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One gene, one enzyme

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Publication date: July 24, 2018

    

The one-gene-one-enzyme hypothesis is the idea put forward in the early 1940s that each gene controls the synthesis or activity of one enzyme. The concept, combining the fields of genetics and biochemistry, was proposed by the American geneticist George Wells Beadle and the American biochemist Edward L. Tatum, who conducted research on Neurospora crassa. Their experiments involved first imaging the form to mutation-inducing X-rays and then culturing it in a minimal growth medium that contained only the essential nutrients needed for the wild-type strain to survive. They found that mutant mold strains require the addition of certain amino acids in order to grow. Using this information, the researchers were able to link mutations in specific genes to the disruption of individual enzymes in the metabolic pathways that would normally produce the missing amino acids. It is now known that not all genes code for an enzyme and that some enzymes are composed of several short polypeptides encoded by two or more genes.

Gene expression is the process by which hereditary information from a gene is converted into a functional product - RNA or protein. Gene expression can be regulated at all stages of the process: during transcription, during translation, and at the stage of post-translational modifications of proteins.

Gene expression is the substrate for evolutionary change.

Regulation of gene expression at the level of transcription in prokaryotes:

Regulation of transcription in cells is carried out at the level of individual genes, their blocks and even whole chromosomes. The ability to control many genes, as a rule, is ensured by the presence of common regulatory nucleotide sequences in them, with which transcription factors of the same type interact. In response to the action of specific effectors, such factors acquire the ability to bind with high precision to regulatory gene sequences. The consequence of this is the weakening or strengthening of the transcription of the corresponding genes. The three main transcriptional steps that are used by bacterial cells to regulate RNA synthesis are initiation, elongation, and termination.

Eukaryotic gene expression differs from that of prokaryotes:

1) Eukaryotes have three types of RNA polymerases: RNA polymerase1 catalyzes the transcription of ribosomal genes. RNA polymerase2 catalyzes the transcription of all structural genes. RNA polymerase3 catalyzes the transcription of tRNA and 5S-ribosomal RNA (catalyses the formation of mRNAs present only in eukaryotes).

2) The promoter region in eukaryotes is longer.

3) In eukaryotes, any gene is represented by alternating coding and non-coding sequences. Coding - exons, non-coding - introns.

4) Eukaryotes have enhancers recognized by proteins. They can be located quite far from the start of transcription. The enhancer and its associated protein approach the RNA polymerase-DNA binding site.

5) There are "silencers" that suppress transcription.

One gene, one enzyme hypothesis, suggests that each gene can encode only one polypeptide chain, which, in turn, can be included as a subunit in a more complex protein complex. The theory was put forward by G. Beadle and E. Tatum in 1941 on the basis of a genetic and biochemical analysis of neurospores, they found that under experimental conditions, under the influence of various mutations, only one of any chain of biochemical reactions was switched off each time. Doubts about the absolute validity of this theory appeared in connection with the discovery of the "two genes - one polypeptide" system, as well as with the existence of overlapping genes. From a functional standpoint, this theory is conditional in connection with the discovery of multifunctional proteins.

Patterns of cell existence in time. Cellular (life) cycle. apoptosis and necrosis. Mitotic (proliferative) cycle. Major events of the mitotic cycle. Reproductive (interphase) and separation (mitosis) phases of the mitotic cycle. Problems of cell proliferation in medicine.

cell cycle- this is the period of existence of a cell from the moment of its formation by dividing the mother cell to its own division or death.

An important component of the cell cycle is mitotic cycle- a complex of interrelated and coordinated in time events occurring in the process of preparing a cell for division and during the division itself. In addition, the life cycle includes the period of performance by the cell of the specific functions of a multicellular organism, as well as periods of rest. During periods of rest, the immediate fate of the cell is not determined: it can either begin preparation for mitosis, or begin specialization in a certain functional direction.

The duration of the mitotic cycle for most cells is from 10 to 50 hours. The biological significance of the mitotic cycle is that it ensures the continuity of chromosomes in a series of cell generations, the formation of cells that are equivalent in volume and content of hereditary information. Thus, the cycle is a general mechanism for the reproduction of the cellular organization of the eukaryotic type in individual development.

consist in the reduplication (self-doubling) of the hereditary material of the mother cell and in the uniform distribution of this material between the daughter cells. According to the two main events of the mitotic cycle in it allocate the reproductive and separation phases corresponding to the interphase and mitosis of classical cytology.

apoptosis- programmed cell death, a regulated process of self-destruction at the cellular level, as a result of which the cell is fragmented into separate apoptotic bodies, limited by the plasma membrane. Fragments of a dead cell are usually very quickly phagocytized by macrophages or neighboring cells, bypassing the development of an inflammatory reaction. The process of apoptosis lasts 1-3 hours. One of the main functions of apoptosis is the destruction of defective (damaged, mutant, infected) cells.

Necrosis- a pathological process, expressed in local tissue death in a living organism as a result of any exogenous or endogenous damage. Necrosis manifests itself in swelling, denaturation and coagulation of cytoplasmic proteins, destruction of cell organelles and, finally, of the entire cell. The most common causes of necrotic tissue damage are: cessation of blood supply and exposure to pathogenic products of bacteria or viruses.

30. Mitotic cycle. The main events of the interphase periods. The content and significance of the phases of mitosis. The biological significance of mitosis.

Mitotic(proliferative)cycle -a complex of interrelated and coordinated events occurring in the process of preparing a cell for division and during division itself. In addition, the life cycle includes cell execution period multicellular organism specific functions as well as dormant periods. During periods of rest, the immediate fate of the cell is not determined: it can either begin preparation for mitosis, or begin specialization in a certain functional direction. The duration of the mitotic cycle for most cells is from 10 to 50 hours.

The biological significance of the mitotic cycle is that it ensures the continuity of chromosomes in a number of cell generations, the formation of cells that are equivalent in volume and content of hereditary information. Thus, the cycle is a general mechanism for the reproduction of the cellular organization of the eukaryotic type in individual development.

Major events of the mitotic cycle are in reduplication(self-doubling) of the hereditary material of the mother cell and in uniform distribution of this material between daughter cells. These events are accompanied by regular changes in the chemical and morphological organization chromosomes - nuclear structures, in which more than 90% of the genetic material of a eukaryotic cell is concentrated (the main part of the extranuclear DNA of an animal cell is located in mitochondria).

Chromosomes, in interaction with extrachromosomal mechanisms, provide: a) storage of genetic information, b) use of this information to create and maintain cellular organization, c) regulation of the reading of hereditary information, d) doubling (self-copying) of genetic material, e) its transfer from the mother cell to the daughter .

Cell changes in the mitotic cycle.

According to the two main events of the mitotic cycle, it is distinguished reproductive and dividing phases corresponding interphase and mitosis classical cytology (Fig. 2.11).

In the initial segment of the interphase ( postmitotic, presynthetic, or Gi-period) the features of the organization of the interphase cell are restored, the formation of the nucleolus, which began in the telophase, is completed. A significant (up to 90%) amount of protein enters the nucleus from the cytoplasm. In the cytoplasm, parallel to the reorganization of the ultrastructure, protein synthesis is intensified. This contributes to the growth of cell mass. If the daughter cell has to enter the next mitotic cycle, the syntheses become directed: chemical precursors of DNA are formed, enzymes that catalyze the DNA reduplication reaction, and a protein is synthesized that starts this reaction. Thus, the processes of preparing the next period of the interphase - the synthetic one - are carried out.

AT synthetic or S-period the amount of hereditary material of the cell doubles. It consists in the divergence of the DNA helix into two chains, followed by the synthesis of a complementary chain near each of them. The result is two identical coils. DNA molecules that are complementary to maternal ones are formed in separate fragments along the length of the chromosome, moreover, non-simultaneously (asynchronously) in different parts of the same chromosome, as well as in different chromosomes. Then parcels (replication units - replicons) of the newly formed DNA are “crosslinked” into one macromolecule.

The time interval from the end of the synthetic period to the beginning of mitosis takes postsynthetic(premitotic), or G 2 - period interphases. It is characterized by intensive synthesis of RNA and especially protein. The doubling of the mass of the cytoplasm is completed in comparison with the beginning of the interphase. This is necessary for the cell to enter mitosis.

The discoveries of the exon-intron organization of eukaryotic genes and the possibility of alternative splicing have shown that the same nucleotide sequence of the primary transcript can provide the synthesis of several polypeptide chains with different functions or their modified analogs. For example, yeast mitochondria contain the box (or cob) gene encoding the cytochrome b respiratory enzyme. It can exist in two forms: The “long” gene, consisting of 6400 bp, has 6 exons with a total length of 1155 bp. and 5 introns. The short form of the gene consists of 3300 bp. and has 2 introns. It is actually a "long" gene devoid of the first three introns. Both forms of the gene are equally well expressed.

After the removal of the first intron of the “long” box gene, based on the combined nucleotide sequence of the first two exons and part of the nucleotides of the second intron, a template for an independent protein, RNA maturase, is formed (Fig. 3.43). The function of RNA maturase is to provide the next stage of splicing - the removal of the second intron from the primary transcript and, ultimately, the formation of a template for cytochrome b.

Another example is a change in the splicing pattern of the primary transcript encoding the structure of antibody molecules in lymphocytes. The membrane form of antibodies has a long "tail" of amino acids at the C-terminus, which ensures the fixation of the protein on the membrane. The secreted form of antibodies does not have such a tail, which is explained by the removal of nucleotides encoding this region from the primary transcript during splicing.

In viruses and bacteria, a situation has been described where one gene can simultaneously be part of another gene, or some DNA nucleotide sequence can be part of two different overlapping genes. For example, on the physical map of the phage FX174 genome (Fig. 3.44), it can be seen that the B gene sequence is located inside the A gene, and the E gene is part of the D gene sequence. This feature of the organization of the phage genome managed to explain the existing discrepancy between its relatively small size (it consists of 5386 nucleotides) and the number of amino acid residues in all synthesized proteins, which exceeds the theoretically permissible for a given genome capacity. The possibility of assembling different peptide chains on mRNA synthesized from overlapping genes (A and B or E and D) is ensured by the presence of ribosomal binding sites within this mRNA. This allows translation of another peptide to start from a new point of reference.

The nucleotide sequence of the B gene is also part of the A gene, and the E gene is part of the D gene.

In the λ phage genome, overlapping genes were also found, translated both with a frameshift and in the same reading frame. It is also assumed that two different mRNAs can be transcribed from both complementary strands of the same DNA region. This requires the presence of promoter regions that determine the movement of RNA polymerase in different directions along the DNA molecule.

The described situations, which testify to the admissibility of reading different information from the same DNA sequence, suggest that overlapping genes are a fairly common element in the organization of the genome of viruses and, possibly, prokaryotes. In eukaryotes, gene discontinuity also allows the synthesis of various peptides based on the same DNA sequence.

With all of this in mind, it is necessary to amend the definition of a gene. Obviously, one can no longer speak of a gene as a continuous sequence of DNA that uniquely encodes a specific protein. Apparently, at present, the formula "One gene - one polypeptide" should still be considered the most acceptable, although some authors suggest changing it: "One polypeptide - one gene." In any case, the term gene should be understood as a functional unit of hereditary material, which by its chemical nature is a polynucleotide and determines the possibility of synthesizing a polypeptide chain, tRNA or rRNA.

One gene one enzyme.

In 1940, J. Beadle and Edward Tatum used a new approach to study how genes provide metabolism in a more convenient object of research - the microscopic fungus Neurospora crassa .. They obtained mutations in which; there was no activity of one or another metabolic enzyme. And this led to the fact that the mutant fungus was not able to synthesize a certain metabolite itself (for example, the amino acid leucine) and could live only when leucine was added to the nutrient medium. The theory "one gene - one enzyme" formulated by J. Beadle and E. Tatum quickly gained wide recognition among geneticists, and they themselves were awarded the Nobel Prize.

Methods. selection of the so-called "biochemical mutations" that lead to disturbances in the action of enzymes that provide different metabolic pathways, have proved to be very fruitful not only for science, but also for practice. First, they led to the emergence of genetics and selection of industrial microorganisms, and then to the microbiological industry, which uses strains of microorganisms that overproduce such strategically important substances as antibiotics, vitamins, amino acids, etc. The principles of selection and genetic engineering of strains of overproducers are based on the notion that "one gene codes for one enzyme". And although this idea is excellent practice brings multi-million dollar profits and saves millions of lives (antibiotics) - it is not final. One gene is not just one enzyme.

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1. A gene is a part of a DNA molecule, which is a functional unit of hereditary information.

1. The gene occupies a certain area in the chromosome - the locus.

2. Recombination can occur within a gene.

3. DNA, which is part of the gene, is capable of repair.

4. There are genes: structural, regulatory, etc.

5. The arrangement of triplets is complementary to amino acids (mutations with a reading frame shift).

6. The genotype, being discrete (consisting of individual genes), functions as a whole.

7. The genetic code is universal.

8. The genetic code is degenerate (for many amino acids there is more than one codon - site)

9. Genes are arranged in a linear order on the chromosome and form a linkage group. The number of linkage groups corresponds to the haploid set of chromosomes (23 in humans or 24 with a reservation for sex - x and y chromosomes).

The structure of proteins is determined by the set and order of amino acids in their peptide chains. It is this sequence of amino acids in peptides that is encrypted in DNA molecules using the genetic code.

Properties of the genetic code:

1. Tripletity- each amino acid is encoded by three adjacent nucleotides.
2. Degeneracy- many amino acids are encrypted with several triplets.
3. Specificity- each triplet can encode only one particular amino acid.
4. Versatility- full compliance of the code in different species of living organisms.
5. Continuity- nucleotide sequences are read triple by triple without gaps.
6. Non-overlapping codons- neighboring triplets do not overlap each other.

20. Ribosomal cycle of protein synthesis (initiation, elongation, termination). Post-translational transformations of proteins.

Hereditary information recorded using the genetic code is stored in DNA molecules, but it does not directly participate in the life support of the cell. The role of an intermediary, whose function is to translate the hereditary information stored in DNA into a working form, is played by RNA. The process of interaction between mRNA and tRNA, which ensures the translation of information from the language of nucleotides into the language of amino acids, is carried out on ribosomes. Ribosomal RNAs are not only a structural component of ribosomes, but also ensure their binding to a specific mRNA nucleotide sequence. This establishes the start and the reading frame during the formation of the peptide chain. In addition, they provide interaction between the ribosome and tRNA. Ribosomes have 2 grooves. One of them holds the growing polypeptide chain, the other mRNA. In addition, 2 tRNA-binding sites are isolated in ribosomes. Aminoacyl-tRNA is located in the aminoacyl, A-site, carrying a specific amino acid. In the peptidyl, P-section, tRNA is usually located. The formation of A- and P-sites is provided by both subunits of the ribosome. Translation can be divided into three phases: initiation, elongation, and termination.

Initiation phase consists in combining two subparticles of the ribosome that were previously separated in the cytoplasm at a certain site of mRNA and attaching the first aminoacyl-tRNA to it. This also sets the frame for reading information contained in mRNA.

elongation phase or elongation of the peptide, includes all reactions from the moment of formation of the first peptide bond to the attachment of the last amk. It is a cyclically recurring event in which there is a specific recognition of the next codon aminoacyl-tRNA located in the A-site, complementary to the link between the anticodon and the codon. Termination phase or the completion of polypeptide synthesis, is associated with the recognition of one of the termination codons (UAA, UAG, UGA) by a specific ribosomal protein when it enters the zone of the A-site of the ribosome. In this case, water is attached to the last amc in the peptide chain and its carboxyl end is separated from the tRNA. As a result, the completed peptide chain loses its connection with the ribosome, which breaks up into two subparticles.

Post-translational transformation of proteins. The peptide chains synthesized during translation, on the basis of their primary structure, acquire a secondary and tertiary, and many a quaternary organization formed by several peptide chains. Depending on the functions performed by proteins, their amino acid sequences can undergo various transformations, forming functionally active protein molecules. Many membrane proteins are synthesized as pre-proteins with a leader sequence at the N-terminus that provides them with membrane recognition. Secretory proteins also have a leader sequence at the N-terminus that ensures their transport across the membrane. Some proteins immediately after translation carry additional amino acid pro-sequences that determine the stability of active protein precursors. During protein maturation, they are removed, allowing the transition of the inactive proprotein to the active protein. Forming a tertiary and quaternary organization in the course of post-translational transformations, proteins acquire the ability to actively function, being included in certain cellular structures and performing enzymatic and other functions.

The relationship between a gene and a trait. The hypothesis "one gene - one enzyme", its modern interpretation: "one gene - one polypeptide chain"

Gene - a section of a DNA molecule that carries information about the structure of a polypeptide chain or macromolecule. The genes of one chromosome are arranged linearly, forming a linkage group. DNA in the chromosome performs different functions. Genes are small in size, although they consist of thousands of base pairs. The presence of a gene is established by the manifestation of the trait of the gene (final product).

For Mendel, the gene is only a symbol convenient for defining the law of inheritance. The relationship between a gene and a trait (product) was discovered while studying fermentation in an airless environment - 1902 Garrod. He studied the pedigrees of patients with alkaptonuria, came to the conclusion that the disease is the result of a violation of nitrogen metabolism, while. Instead of urea, a dark substance is formed. With the assistance of Bats in 1908, it was suggested that the disease occurs in recessive homozygotes who lack some kind of enzymatic reaction, which leads to the accumulation and excretion of the substrate, which normally should have been split. Human blood contains homogentisic acid, but normally it is broken down by homogentisic acid oxidase to maleic acetate, then to water and carbon dioxide. Patients do not have oxidase, so acid accumulates and is excreted in the urine.

Albinism is also inherited, although it is much more common. In this disease, there is no enzyme that converts tyrosine to melanin.

Until 1940, the opinion of scientists was divided, but there was no theory. 1940 - Beadle and Tatum hypothesized: 1 gene - 1 enzyme. E that hypothesis played an important role - scientists began to consider the final products. It turned out that the hypothesis has limitations, because All enzymes are proteins, but not all proteins are enzymes. As a rule, proteins are oligomers - i.e. exist in a quaternary structure. For example, a tobacco mosaic capsule has over 1200 polypeptides. At present, the most acceptable hypothesis is "One gene - one polypeptide". The term gene should be understood as a functional unit of heredity, which by its chemical nature is a polynucleotide and determines the possibility of synthesizing a polypeptide chain.

Gene as a unit of variability. Gene mutations and their classification. Causes and mechanisms of gene mutations. Consequences of gene mutations.

A gene is an elementary unit of hereditary material. Gene mutations associated with a change in the internal structure of genes, which turns one allele into another.

Changes in the structure of the DNA that forms the gene can be divided into 3 groups.

4.2.1. One gene, one enzyme hypothesis

First research. After in 1902 Garrod pointed out the connection of a genetic defect in alkaptonuria with the body's inability to break down homogentisic acid, it was important to elucidate the specific mechanism underlying this disorder. Since then it was already known that metabolic reactions are catalyzed by enzymes, it could be assumed that it was the violation of some enzyme that leads to alkaptonuria. Such a hypothesis was discussed by Driesch (in 1896). It was also expressed by Haldane (1920, see) and Garrod (1923). Important stages in the development of biochemical genetics were the work of Kuhn and Butenandt on the study of eye color in the mill moth. Ephesia kuhniella and similar studies by Beadle and Ephrussi on Drosophila(1936). In these pioneering works, insect mutants previously studied by genetic methods were selected to elucidate the mechanisms of action of genes. However, this approach did not lead to success. The problem turned out to be too complicated, and in order to solve it, it was necessary:

1) choose a simple model organism convenient for experimental study;

2) to look for the genetic basis of biochemical traits, and not the biochemical basis of genetically determined traits. Both conditions were met by Beadle and Tatum in 1941 (see also Beadle 1945).

Beadle and Tatum model. Their article began like this:

“From the point of view of physiological genetics, the development and functioning of an organism can be reduced to a complex system of chemical reactions that are somehow controlled by genes. It is quite logical to assume that these genes ... either act as enzymes themselves, or determine their specificity. It is known that genetic physiologists usually try to investigate the physiological and biochemical foundations of already known hereditary traits. This approach made it possible to establish that many biochemical reactions are controlled by specific genes. Such studies have shown that enzymes and genes have the same order of specificity. However, the scope of this approach is limited. The most serious limitation is that, in this case, hereditary traits that do not have a lethal effect and, therefore, are associated with reactions that are not very essential for the life of the organism, fall into the field of view of researchers. The second difficulty ... is that the traditional approach to the problem involves the use of outwardly manifest signs. Many of them are morphological variations based on systems of biochemical reactions so complex that their analysis is extremely difficult.

These considerations led us to the following conclusion. The study of the general problem of genetic control of biochemical reactions that determine development and metabolism should be carried out using procedure opposite to the generally accepted: instead of trying to find out the chemical basis of known hereditary traits, it is necessary to establish whether genes control known biochemical reactions and how they do it. The ascomycete neurospore has the properties that make it possible to implement this approach and, at the same time, serves as a convenient object for genetic studies. That is why our program was built on the use of this particular organism. We proceeded from the fact that X-ray exposure causes mutations in the genes that control certain chemical reactions. Suppose that in order to survive in a given environment, the organism must carry out some kind of chemical reaction, then a mutant deprived of such an ability will turn out to be unviable under these conditions. However, it can be maintained and studied if grown in a medium to which the vital product of a genetically blocked reaction has been added.”

4 Action of genes 9

Rice. 4.1. Scheme of the experiment for the detection of biochemical neurospore mutants On a complete medium, mutations induced by X-rays or ultraviolet do not interfere with the growth of the fungus. However, the mutant does not grow on minimal medium. When vitamins are added to the minimal medium, the ability to grow is restored When amino acids are added, there is no growth Based on these data, it can be assumed that the mutation occurred in the gene that controls the metabolism of the vitamin The next step is to identify the vitamin that can restore normal function The genetic block was found among the reactions of vitamin biosynthesis .

Next, Beadle and Tatum describe the design of the experiment (Figure 4.1). The composition of the complete medium included agar, inorganic salts, malt extract, yeast extract and glucose. The minimal medium contained only agar, salts, biotin, and a carbon source. The mutants that grew on the complete medium and did not grow on the minimal medium were studied in the most detail. In order to establish the compound, the synthesis of which was impaired in each of the mutants, individual components of the complete medium were added to the minimal agar.

In this way, strains were isolated that were unable to synthesize certain growth factors: pyridoxine, thiamine, and para-aminobenzoic acid. These defects have been shown to be due to mutations at specific loci. The work marked the beginning of numerous studies on neurospores, bacteria and yeasts, in which a correspondence was established between the "genetic blocks" responsible for individual metabolic steps and specific enzyme disorders. This approach has rapidly evolved into a tool for researchers to uncover metabolic pathways.

The hypothesis "one gene - one enzyme" has received strong experimental confirmation. As the work of subsequent decades showed, it proved to be surprisingly fruitful. Analysis of defective enzymes and their normal variants soon made it possible to identify a class of genetic disorders that led to a change in the function of the enzyme, although the protein itself was still detectable and retained immunological properties. In other cases, the temperature optimum of enzyme activity changed. Some variants could be explained by a mutation that affects the general regulatory mechanism and, as a result, changes the activity of a whole group of enzymes. Such studies led to the creation of the concept of regulation of gene activity in bacteria, which included the concept of the operon.

10 4. Action of genes

The first examples of enzymatic disorders in humans. The first human hereditary disease for which an enzymatic disorder could be shown was methemoglobinemia with a recessive mode of inheritance (Gibson and Harrison, 1947; Gibson, 1948) (25080). In this case, the damaged enzyme is NADH - dependent methemoglobin reductase. The first attempt to systematically study a group of human diseases associated with metabolic defects was made in 1951. In a study of glycogen storage disease, the Corys showed that in eight out of ten cases of a pathological condition that was diagnosed as Gierke's disease (23220), the structure of liver glycogen was a normal variant, and in two cases it was clearly disturbed. It was also evident that liver glycogen, accumulating in excess, could not be directly converted into sugar, since patients tend to hypoglycemia. Many enzymes are needed to break down glycogen into glucose in the liver. Two of them, amyl-1,6-glucosidase and glucose-6-phosphatase, were chosen for study as possible defective elements of the enzyme system. Phosphate release from glucose-6phosphate was measured in liver homogenates at various pH values. The results are presented in fig. 4.2. In a normal liver, high activity was found with an optimum at pH 6-7. Severe liver dysfunction in cirrhosis correlated with a slight decrease in activity. On the other hand, in the case of Gierke's disease with a fatal outcome, the activity of the enzyme could not be detected at all; the same result was obtained in the examination of the second similar patient. In two patients with less severe symptoms, there was a significant decrease in activity.

It was concluded that in these cases of Gierke's disease with a fatal outcome, there was a defect in glucose-6-phosphatase. However, in most of the milder cases, the activity of this enzyme was not lower than in liver cirrhosis, and only in two patients was it slightly lower (Fig. 4.2).

According to the Corey spouses, the abnormal accumulation of glycogen in muscle tissue cannot be associated with a lack of glucose-6-phosphatase, since this enzyme is absent in the muscles and is normal. As a possible explanation for muscle glycogenosis, they suggested a violation of the activity of amylo-1,6-glucosidase. This prediction was soon confirmed: Forbes discovered such a defect in one of the clinically significant cases of glycogen storage disease involving the heart and skeletal muscles. Now we

4. Action of genes 11

a large number of enzymatic defects are known in glycogen storage disease.

Although the various forms of this disease vary somewhat in degree of manifestation, there is much in common between them clinically. With one exception, they are all inherited in an autosomal recessive manner. If enzymatic defects had not been uncovered, the pathology of glycogen accumulation would be considered as a single disease with characteristic intrafamilial correlations in severity, symptom details, and timing of death. Thus, we have an example where genetic heterogeneity, which could only be assumed on the basis of the study of the phenotype (Section 3.3.5), was confirmed by analysis at the biochemical level: the study of enzymatic activity made it possible to identify specific genes.

In subsequent years, the pace of research into enzymatic defects increased, and for the 588 identified recessive autosomal genes that McKusick describes in the sixth edition of his book Mendelian Inheritance in Man (1983), more than 170 cases were found to have specific enzymatic disorders. Our progress in this area is directly related to the development of the concepts and methods of molecular genetics.

Some stages of the study of enzymatic disorders in humans. We present only the most important milestones in this ongoing process: 1934 Völling discovered phenylketonuria

1941 Beadle and Tatum formulated the one-gene-one-enzyme hypothesis 1948 Gibson described the first case of an enzymatic disorder in a human disease (recessive methemoglobinemia)

1952 Cory's discovered glucose-6-phosphatase deficiency in Gierke's disease

1953 Jervis demonstrated the absence of phenylalanine hydroxylase in phenylketonuria. Bickel reported the first attempt to alleviate an enzymatic disorder by adopting a diet low in phenylalanine.

1955 Smithies developed the starch gel electrophoresis technique

1956 Carson et al. discovered a defect in glucose-6-phosphate dehydrogenase (G6PD) in a case of induced hemolytic anemia

1957 Kalkar et al. described enzymatic deficiency in galactosemia, showing that humans and bacteria have an identical enzymatic disorder

1961 Krut and Weinberg demonstrated an enzyme defect in galactosemia in vitro in cultured fibroblasts

1967 Sigmiller et al. discovered a hypoxanthine-guanine phosphoribosyltransferase (HPRT) defect in Lesch-Nyhan syndrome

1968 Cleaver described violation of excisional repair in xeroderma pigmentosa

1970 Neufeld identified enzymatic defects in mucopolysaccharidoses, which made it possible to identify the pathways for the breakdown of mucopolysaccharides

1974 Brown and Goldstein proved that the genetically determined overproduction of hydroxymethylglutaryl-CoA reductase in familial hypercholesterolemia is due to a defect in the membrane-located low-density lipoprotein receptor, which modulates the activity of this enzyme (HMG)

1977 Sly et al. demonstrated that mannose-6-phosphate (as a component of lysosomal enzymes) is recognized by fibroblast receptors. A genetic defect in processing prevents the binding of lysosomal enzymes, resulting in impaired release into the cytoplasm and subsequent secretion into the plasma (I-cell disease)

12 4. Action of genes

1980 In pseudohypoparathyroidism, a defect in the protein that provides the coupling of the receptor and cyclase was discovered.