Regulatory protein. Course of lectures on general biochemistry
Proteins involved in the regulation of metabolism can themselves serve as ligands (for example, peptide hormones), i.e., interact with other proteins, such as hormonal receptors, exerting a regulatory effect. Other regulatory proteins such as hormone receptors or the regulatory subunit of protein kinase (a cAMP-activated enzyme) have activities controlled by the binding of regulatory ligands (i.e., hormones and cAMP, respectively) (see Chapter 4). In order for the activities of proteins of this class to be specifically regulated by ligands, such molecules must first of all have sites that specifically (and, as a rule, with high affinity) bind the ligand, which gives the molecules the ability to distinguish the ligand from others chemical compounds. In addition, the protein must have such a structure that, as a result of ligand binding, its conformation can change, i.e., provide the possibility of exerting a regulatory effect. For example, in mammals, the specific binding of cAMP to the regulatory subunit of individual protein kinases leads to a decrease in the binding affinity of this subunit to the catalytic subunit of the enzyme (see Chapter 4). This causes the dissociation of both protein subunits of the enzyme. The catalytic subunit, freed from the inhibitory action of the regulatory subunit, is activated and catalyzes the phosphorylation of proteins. Phosphorylation changes the properties of certain proteins, which affects processes under the control of cAMP. The interaction of steroid hormones with their receptors causes conformational changes in the latter that give them the ability to bind to the cell nucleus (see Chapter 4). This interaction also changes other properties of the receptors that are important in mediating the effect of steroid hormones on the transcription of certain types of mRNA.
In order to have such specialized and highly specific functions, proteins, as a result of the evolution of genes that determine their amino acid sequence, had to acquire the structure that they currently have. In some cases, other genes encoding the synthesis of products that modify the regulatory proteins themselves (for example, by glycosylation) also take part in the process. Since gene evolution appears to have occurred through mechanisms such as mutation of preexisting genes and recombination of regions of different genes (as discussed), this placed certain constraints on protein evolution. From an evolutionary perspective, it would probably be easier to modify existing structures than to create entirely new genes. In this regard, the existence of some homology in the amino acid sequences of different proteins may not be unexpected, since their genes could arise due to the evolution of common precursors. Since, as noted above, regions of proteins adapted to bind regulatory ligands, such as cAMP and steroids or their analogues, should already have existed by the time these ligands appeared, it is easy to imagine how modification of the genes of such proteins could lead to the synthesis of other proteins , maintaining high specificity of regulatory ligand binding.
In Fig. Figure 2-2 shows one of the hypothetical schemes for the evolution of primitive glucotransferase into three existing types of regulatory proteins: bacterial cAMP-binding protein (CAP or CRP), which regulates the transcription of several genes encoding enzymes that take part in lactose metabolism, as well as cAMP-binding protein mammals, which regulates the activity of cAMP-dependent protein kinase, which mediates the action of cAMP in humans (see Chapter 4), and adenylate cyclase (see Chapter 4). In relation to bacterial protein and kinase, the ATP-binding regions of primitive glucokinase have evolved to acquire greater cAMP binding specificity. The bacterial protein also acquired additional polynucleotide (DNA)-binding ability. The evolution of the kinase involves the acquisition of glucophosphotransferase ability to phosphorylate proteins. Finally, adenylate cyclase could be formed from glucokinase by replacing the ADP-generating function with a cAMP-generating one. These conclusions cannot but be purely hypothetical; nevertheless, they show how the molecular evolution of the listed regulatory proteins could have occurred.
Rice. 2-2. Proposed origin of cAMP-dependent protein kinase, adenylate cyclase and bacterial cAMP-binding regulatory protein (Baxter, MacLeod).
Although many details are missing from the picture of protein evolution, the current knowledge of the structure of proteins and genes provides some basis for analyzing the question of whether the genes for some polypeptide hormones evolved from a common precursor gene. Individual polypeptide hormones can be grouped according to structural similarity. It is not surprising that hormones belonging to the same group may have similar physiological effects they cause, as well as a similar mechanism of action. Thus, growth hormone (GH), prolactin and chorionic somatomammotropin (placental lactogen) are characterized by a high degree of amino acid sequence homology. Glycoprotein hormones - thyroid-stimulating hormone (TSH), human chorionic gonadotropin (hCG), follicle-stimulating (FSH) and luteinizing (LH) hormones - consist of two subunits, each of which (A-chain) is identical or almost identical in all hormones of a given groups . The amino acid sequence of the B subunits in different hormones, although not identical, has structural homology. It is probably these differences in B-chains that have crucial to impart specificity to the interaction of each hormone with its target tissue. Insulin shows several structural analogues and shares biological activity with other growth factors, such as somatomedin and non-inhibitable insulin-like activity (NIPA).
As for the group of hormones to which growth hormone belongs, the nucleotide sequence of the mRNAs encoding their synthesis has been partially elucidated. Each amino acid requires three nucleotides in the DNA (and therefore in the mRNA transcribed from it). Although this triplet of nucleotides; (codon) corresponds to this particular amino acid; there can be several codons for the same amino acid. Such "degeneracy" genetic code determines the possibility of greater or lesser homology of the nucleotide sequences of these two genes, which determine the structure of the two hormones, than is found in proteins. Thus, if two proteins have random amino acid sequence homology, then the sequences nucleic acids could show large differences. However, with regard to genes encoding the synthesis of hormones of the somatotropin group, this is not the case; The homology of a nucleic acid sequence is higher than that of an amino acid sequence. Human growth hormone and human chorionic somatomammotropin, which have 87% amino acid sequence homology, have 93% nucleic acid sequence homology in their mRNAs. Human and rat growth hormones share 70% amino acid sequence homology, and their mRNAs exhibit 75% nucleic acid sequence homology. In some regions of the mRNA of rat growth hormone and human chorionic somatomammotropin (mRNA of two different hormones in two biological species), the homology is 85% (Fig. 2-3). Thus, only minimal base changes in DNA cause hormone differences. Therefore, these data support the conclusion that the genes for such hormones were formed during evolution from a common predecessor. From the standpoint of the presented ideas about symbols and the reactions they cause, it is significant that each of the three hormones of this group has an effect on growth (see below). Growth hormone is a factor that determines linear growth. Prolactin plays important role in the processes of lactation and thereby ensures the growth of the newborn. Human chorionic somatomammotropin, although its physiological significance has not been precisely established, can have a significant effect on intrauterine growth by directing nutrients entering the mother's body to the growth of the fetus.
Such as hormone receptors or the regulatory subunit of protein kinase (a cAMP-activated enzyme) have activities that control the binding of regulatory ligands (i.e., hormones and cAMP, respectively). In order for the activities of proteins of this class to be specifically regulated by ligands, such molecules must first of all have sites that specifically (and, as a rule, with high affinity) bind the ligand, which gives the molecules the ability to distinguish ligands from other chemical compounds. In addition, the protein must have such a structure that, as a result of ligand binding, its conformation can change, i.e. provide the ability to provide regulatory action. For example, in mammals, the specific binding of cAMP to the regulatory subunit of individual protein kinases leads to a decrease in the binding affinity of this subunit with the catalytic subunit of the enzyme. This causes the dissociation of both protein subunits of the enzyme. The catalytic subunit, freed from the inhibitory action of the regulatory subunit, is activated and catalyzes the phosphorylation of proteins. Phosphorylation changes the properties of certain proteins, which affects processes under the control of cAMP.
As for the group of hormones to which growth hormone belongs, the nucleotide sequence of the mRNA encoding their synthesis has been partially identified (Baxter J.D. ea, 1979). Each amino acid requires three nucleotides in the DNA (and therefore in the mRNA transcribed from it). Although a given triplet of nucleotides (codon) corresponds to a given amino acid, there can be several codons for the same amino acid. This “degeneracy” of the genetic code determines the possibility of greater or lesser homology of the nucleotide sequences of these two genes, which determine the structure of the two hormones, than is found in proteins. Thus, if two proteins have random amino acid sequence homology, then the nucleic acid sequences could show large differences. However, with regard to genes encoding the synthesis of hormones of the somatotropin group, this is not the case; The homology of the nucleic acid sequence is higher than the homology of the amino acid sequence (Baxter J.D. ea, 1979). Human growth hormone and human chorionic somatomammotropin, which have 87% amino acid sequence homology, have 93% nucleic acid sequence homology in their mRNAs. Human and rat growth hormones share 70% amino acid sequence homology, and their mRNAs exhibit 75% nucleic acid sequence homology. In some regions of the mRNA of rat growth hormone and human chorionic somatomammotropin (mRNA of two different hormones in two biological species), the homology is 85%. Thus, only minimal base changes in DNA cause hormone differences. Therefore, these data support the conclusion that the genes for such hormones were formed during evolution from a common predecessor. From the standpoint of the presented ideas about symbols and the reactions they cause, it is significant that each of the three hormones of this group has an effect on growth. Growth hormone is a factor that determines linear growth. Prolactin plays an important role in lactation processes and thereby ensures the growth of the newborn. Human chorionic somatomammotropin, although its physiological significance is not precisely established, can have a significant effect on intrauterine growth by directing nutrients entering the mother's body that affect fetal growth (
Regulatory proteins(from Lat. regulo - putting in order, putting in order), group . involved in the regulation of various biochem. processes. An important group of regulatory proteins that this article is devoted to are proteins that interact with DNA and control (expression in the characteristics and properties of the organism). The vast majority of these regulatory proteins function at the level transcriptions(synthesis of messenger RNA, or mRNA, on a DNA template) and is responsible for the activation or repression (suppression) of mRNA synthesis (respectively, activator proteins and repressor proteins).
Known approx. 10 repressors. Naib. among them, repressors of prokaryotes (bacteria, blue-green algae), regulating the synthesis of enzymes involved in (lac-repressor) in Escherichia coli (E.coli), and the repressor of bacteriophage A, have been studied. Their action is realized by binding to specific. DNA sections (operators) of the corresponding genes and blocking the initiation of the mRNA encoded by these genes.
A repressor is usually a dimer of two identical polypeptide chains oriented in mutually opposite directions. Repressors physically prevent RNA polymerase join DNA in the promoter region (the binding site of the DNA-dependent RNA polymerase enzyme that catalyzes the synthesis of mRNA on a DNA template) and begin the synthesis of mRNA. It is assumed that the repressor only interferes with initiation and does not affect mRNA elongation.
A repressor can control the synthesis of cells. one protein or a whole series. the expression of which is coordinated. As a rule, these are enzymes that serve one metabolite. path; their genes are part of one (a set of interconnected genes and adjacent regulatory regions).
Mn. repressors can exist in both active and inactive forms, depending on whether or not they are associated with inducers or corepressors (respectively, substrates in the presence of which the rate of synthesis of a particular enzyme is specifically increased or decreased; see Regulators); these interactions have a non-covalent nature.
For efficient gene expression, it is necessary not only for the repressor to be inactivated by the inducer, but also to be realized specifically. positive switch-on signal, which is mediated by regulatory proteins working “in tandem” with the cyclic. adenosine monophosphate (cAMP). The latter binds to specific regulatory proteins (the so-called CAP catabolite gene activator protein, or protein catabolism activator-BAK). This is a dimer with a pier. m. 45 thousand. After binding to cAMP, it acquires the ability to attach to specific. areas on DNA, dramatically increasing the efficiency of the genes of the corresponding operon. In this case, CAP does not affect the rate of growth of the mRNA chain, but controls the stage of transcription initiation - the attachment of RNA polymerase to the promoter. In contrast to the repressor, CAP (in complex with cAMP) facilitates the binding of RNA polymerase to DNA and makes initiation events more frequent. The site where CAP attaches to DNA is adjacent directly to the promoter on the side opposite to where the operator is localized.
Positive regulation (for example, the lac operon of E. coli) can be described by a simplified scheme: with a decrease in the concentration of glucose (the main carbon source), the concentration of cAMP increases, which binds to the CAP, and the resulting complex with the lac promoter. As a result, the binding of RNA polymerase to the promoter is stimulated and the speed of genes that encode enzymes that allow the cell to switch to the use of another carbon source, lactose, increases. There are other special regulatory proteins (for example, protein C), the functioning of which is described by a more complex scheme; they control a narrow range of genes and can act as both repressors and activators.
Repressors and operon-specific activators do not affect the specificity of the RNA polymerase itself. This last level of regulation is implemented in cases involving mass. change in the spectrum of expressed genes. Thus, in E.coli, genes encoding heat shock proteins, which are expressed under a number of stress conditions of the cell, are read by RNA polymerase only when a special regulatory protein, the so-called. factor s 32. A whole family of these regulatory proteins (s-factors), which change the promoter specificity of RNA polymerase, have been found in bacilli and other bacteria.
Dr. a type of regulatory protein changes the catalytic. properties of RNA polymerase (so-called antiterminator proteins). Thus, two such proteins are known in bacteriophage X, which modify the RNA polymerase so that it does not obey cellular termination signals (this is necessary for the active expression of phage genes).
General scheme of genetic control, including the functioning of regulatory proteins, also applies to bacteria and eukaryotic cells (all organisms, with the exception of bacteria and blue-green algae).
Eukaryotic. cells respond to external signals (for them these are, for example, hormones) are basically the same as bacterial cells react to changes in nutritional concentration. substances in environment, i.e. by reversible repression or activation (derepression) of individual genes. At the same time, regulatory proteins that simultaneously control the activity large number genes, can be used in various combinations. Similar combination genetic. regulation can provide differentiation. the development of the entire complex multicellular organism thanks to the interaction. relatively small number of key regulatory proteins
The system for regulating gene activity in eukaryotes has additional components. a level not present in bacteria, namely the translation of all nucleosomes (repeating subunits chromatin), included in the transcription unit into an active (decondensed) form in those cells where this gene should be functionally active. It is assumed that a set of specific regulatory proteins that have no analogues in prokaryotes is involved here. These proteins not only recognize specifically. sections of chromatin (or DNA), but also cause certain structural changes in adjacent areas. Regulatory proteins like bacterial activators and repressors appear to be involved in the downstream regulation of individual genes in the activator regions. chromatin.
A broad class of regulatory proteins eukaryotes-receptor proteins steroid hormones.
Author Chemical encyclopedia b.b. N.S. ZefirovREGULATORY PROTEINS(from Latin regulo - put in order, establish), a group of proteins involved in the regulation of various biochemical processes. An important group of REGULATORY PROTEINS, which this article is devoted to, are proteins that interact with DNA and control gene expression (gene expression in the characteristics and properties of an organism). The vast majority of these REGULATORY PROTEINS b. functions at the transcription level (synthesis of messenger RNA, or mRNA, on a DNA template) and is responsible for activation or repression (suppression) of mRNA synthesis (activator proteins and repressor proteins, respectively).
About 10 repressors are known. Naib. among them, repressors of prokaryotes (bacteria, blue-green algae), regulating the synthesis of enzymes involved in lactose metabolism (lac repressor) in Escherichia coli (E. coli), and a repressor of bacteriophage A, were studied. Their action is realized by binding to specific DNA regions (operators) of the corresponding genes and blocking the initiation of transcription of the mRNA encoded by these genes.
A repressor is usually a dimer of two identical polypeptide chains oriented in mutually opposite directions. Repressors physically prevent RNA polymerase from joining DNA at the promoter region (the binding site for the DNA-dependent RNA polymerase enzyme that catalyzes the synthesis of mRNA on a DNA template) and begin mRNA synthesis. It is assumed that the repressor only interferes with the initiation of transcription and does not affect the elongation of mRNA.
A repressor can control the synthesis of cells. one protein or a number of proteins whose expression is coordinated. As a rule, these are enzymes that serve one metabolite. path; their genes are part of one operon (a set of interconnected genes and adjacent regulatory regions).
Mn. repressors can exist in both active and inactive forms, depending on whether they are associated or not with inducers or corepressors (respectively, substrates, in the presence of which the rate of synthesis of a particular enzyme is specifically increased or decreased; see Enzyme regulators); these interactions are of a non-covalent nature.
For efficient gene expression, it is necessary not only for the repressor to be inactivated by the inducer, but also for the specific expression to be realized. a switching signal that is mediated by REGULATORY PROTEINS b., working “paired” with cyclic adenosine monophosphate (cAMP). The latter binds to specific REGULATORY PROTEINS b. (so-called CAP protein-catabolite gene activator, or protein catabolism activator-BAK). This is a dimer with molecular weight 45 thousand After binding to cAMP, it acquires the ability to attach to specific sites on DNA, sharply increasing the efficiency of transcription of the genes of the corresponding operon. In this case, CAP does not affect the rate of growth of the mRNA chain, but controls the stage of transcription initiation - the attachment of RNA polymerase to the promoter. In contrast to the repressor, CAP (in complex with cAMP) facilitates the binding of RNA polymerase to DNA and makes transcription initiation more frequent. The site where CAP attaches to DNA is adjacent directly to the promoter on the side opposite to where the operator is localized.
Positive regulation (for example, the lac operon of E. coli) can be described by a simplified scheme: with a decrease in the concentration of glucose (the main carbon source), the concentration of cAMP increases, which binds to the CAP, and the resulting complex with the lac promoter. As a result, the binding of RNA polymerase to the promoter is stimulated and the rate of transcription of genes that encode enzymes that allow the cell to switch to the use of another carbon source, lactose, increases. There are other special REGULATORY PROTEINS b. (for example, protein C), the functioning of which is described by a more complex scheme; they control a narrow range of genes and can act as both repressors and activators.
Repressors and operon-specific activators do not affect the specificity of the RNA polymerase itself. This last level of regulation is implemented in cases involving mass. change in the spectrum of expressed genes. Thus, in E. coli, genes encoding heat shock proteins, which are expressed under a number of stress conditions of the cell, are read by RNA polymerase only when a special REGULATORY PROTEINS factor, called factor s 32, is included in its composition. A whole family of these REGULATORY PROTEINSb. (s-factors) that change the promoter specificity of RNA polymerase have been found in bacilli and other bacteria.
Dr. variety REGULATORY PROTEINSb. changes the catalytic properties of RNA polymerase (called antiterminator proteins). Thus, two such proteins are known in bacteriophage X, which modify RNA polymerase so that it does not obey cellular signals for transcription termination (this is necessary for the active expression of phage genes).
General scheme of genetic control, including the functioning of REGULATORY PROTEINS, also applies to bacteria and eukaryotic cells (all organisms, with the exception of bacteria and blue-green algae).
Eukaryotic. cells respond to external signals (for them, these are, for example, hormones) in principle in the same way as bacterial cells react to changes in the concentration of nutrition. substances in the environment, i.e. by reversible repression or activation (derepression) of individual genes. At the same time, REGULATORY PROTEINS, which simultaneously control the activity of a large number of genes, can be used in various combinations. Similar combination genetic. regulation can provide differentiation. development of an entire complex multicellular organism due to the interaction of a relatively small number of key REGULATORY PROTEINS b.
In the system of regulation of gene activity in eukaryotes there is an additional level that is absent in bacteria, namely, the translation of all nucleosomes (repeating chromatin subunits) that are part of the transcription unit into an active (decondensed) form in those cells where this gene should be functionally active . It is assumed that a set of specific REGULATORY PROTEINS is involved here, which have no analogues in prokaryotes. These proteins not only recognize specific areas of chromatin (or DNA), but also cause certain structural changes in adjacent areas. REGULATORY PROTEINS, similar to bacterial activators and repressors, appear to be involved in the regulation of subsequent transcription of individual genes in the activating regions. chromatin.
Extensive class REGULATORY PROTEINSb. eukaryotes-receptor proteins of steroid hormones.
Amino acid sequence REGULATORY PROTEINSb. encoded by so-called regulatory genes. Mutational inactivation of the repressor leads to uncontrolled synthesis of mRNA, and, consequently, of a certain protein (as a result of translation-protein synthesis on the mRNA matrix). Such organisms are called constitutive mutants. Loss of the activator as a result of mutation leads to a persistent decrease in the synthesis of the regulated protein.
Literature: Strayer L., Biochemistry, trans. from English, vol. 3, M., 1985, p. 112-25.
P.L. Ivanov.
Chemical encyclopedia. Volume 4 >>
More and more regulatory proteins are constantly being discovered; at present, probably only a small part of them is known.
There are several types of proteins that perform a regulatory function:
- receptor proteins that perceive the signal;
- signaling proteins-hormones and other substances that carry out intercellular signaling (many of them, although by no means all, are proteins or peptides);
- regulatory proteins that regulate many processes inside cells.
Proteins involved in intercellular signaling
Hormone proteins (and other proteins involved in intercellular signaling) influence metabolism and other physiological processes.
Hormones- these are substances that are formed in the endocrine glands, are transported by the blood and carry an information signal. Hormones spread without targeting and act only on those cells that have suitable receptor proteins. Hormones bind to specific receptors. Hormones usually regulate slow processes, for example, the growth of individual tissues and the development of the body, but there are exceptions: for example, adrenaline is a stress hormone, a derivative of amino acids. It is released when a nerve impulse impacts the adrenal medulla. At the same time, the heart begins to beat faster, blood pressure rises and other responses occur. It also acts on the liver (breaks down glycogen). Glucose is released into the blood and used by the brain and muscles as a source of energy.
Receptor proteins
Proteins with a regulatory function also include receptor proteins. Membrane receptor proteins transmit a signal from the cell surface inside, transforming it. They regulate cell functions by binding to a ligand that “sits” on this receptor outside the cell; as a result, another protein inside the cell is activated.
Most hormones act on a cell only if there is a certain receptor on its membrane - another protein or glycoprotein. For example, the β2-adrenergic receptor is located on the membrane of liver cells. Under stress, the adrenaline molecule binds to the β2-adrenergic receptor and activates it. Next, the activated receptor activates the G protein, which attaches GTP. After many intermediate steps of signal transduction, glycogen phosphorolysis occurs. The receptor carried out the very first operation of transmitting a signal leading to the breakdown of glycogen. Without it, there would be no subsequent reactions within the cell.
Intracellular regulatory proteins
Proteins regulate processes occurring inside cells using several mechanisms:
- interactions with DNA molecules (transcription factors);
- by phosphorylation (protein kinase) or dephosphorylation (protein phosphatase) of other proteins;
- through interaction with the ribosome or RNA molecules (translation regulation factors);
- influence on the process of intron removal (splicing regulation factors);
- influence on the rate of decay of other proteins (ubiquitins, etc.).
Transcription regulator proteins
Transcription factor is a protein that, when entering the nucleus, regulates DNA transcription, that is, reading information from DNA to mRNA (synthesis of mRNA using a DNA template). Some transcription factors change the structure of chromatin, making it more accessible to RNA polymerases. There are various auxiliary transcription factors that create the desired DNA conformation for the subsequent action of other transcription factors. Another group of transcription factors are those factors that do not bind directly to DNA molecules, but are combined into more complex complexes using protein-protein interactions.
Translation regulation factors
Broadcast- synthesis of polypeptide chains of proteins using an mRNA matrix, performed by ribosomes. Translation regulation can be carried out in several ways, including with the help of repressor proteins that bind to mRNA. There are many cases where the repressor is a protein that is encoded by this mRNA. In this case, regulation occurs according to the type feedback(an example of this is the repression of the synthesis of the enzyme threonyl-tRNA synthetase).
Factors regulating splicing
There are regions within eukaryotic genes that do not code for amino acids. These regions are called introns. They are first copied onto pre-mRNA during transcription, but then cut out by a special enzyme. This process of removing introns and then joining the ends of the remaining sections together is called splicing. Splicing is carried out by small RNAs, usually associated with proteins called splicing regulatory factors. Proteins with enzymatic activity take part in splicing. They give the pre-mRNA the desired conformation. Assembly of the complex (spliceosome) requires energy consumption in the form of split ATP molecules, therefore this complex contains proteins that have ATPase activity.
Alternative splicing exists. Splicing features are determined by proteins that can bind to the RNA molecule in intronic regions or areas at the exon-intron boundary. These proteins can prevent the removal of some introns and at the same time promote the excision of others. Targeted regulation of splicing can have significant biological consequences. For example, in a fruit fly