Protein synthesis in muscle cells. Protein synthesis in muscle cells What is protein synthesis in a cell

The totality of reactions of biological synthesis is called plastic exchange, or assimilation. The name of this type of exchange reflects its essence: from simple substances entering the cell from the outside, substances similar to the substances of the cell are formed.

Consider one of the most important forms of plastic metabolism - protein biosynthesis. The whole variety of properties of proteins is ultimately determined by the primary structure, i.e., the sequence of amino acids. A huge number of unique combinations of amino acids selected by evolution are reproduced by the synthesis of nucleic acids with such a sequence of nitrogenous bases that corresponds to the amino acid sequence in proteins. Each amino acid in the polypeptide chain corresponds to a combination of three nucleotides - a triplet.

The process of realization of hereditary information in biosynthesis is carried out with the participation of three types of ribonucleic acids: informational (matrix) - mRNA (mRNA), ribosomal - rRNA and transport - tRNA. All ribonucleic acids are synthesized in the corresponding regions of the DNA molecule. They are much smaller than DNA and are a single chain of nucleotides. Nucleotides contain a phosphoric acid residue (phosphate), a pentose sugar (ribose) and one of the four nitrogenous bases - adenine, cytosine, guanine and uracil. The nitrogenous base, uracil, is complementary to adenine.

The process of biosynthesis is complex and includes a number of steps - transcription, splicing and translation.

The first stage (transcription) occurs in the cell nucleus: mRNA is synthesized at the site of a certain gene of the DNA molecule. This synthesis is carried out with the participation of a complex of enzymes, the main of which is DNA-dependent RNA polymerase, which attaches to the initial (initial) point of the DNA molecule, unwinds the double helix and, moving along one of the strands, synthesizes a complementary strand of mRNA next to it. As a result of transcription, mRNA contains genetic information in the form of a sequential alternation of nucleotides, the order of which is exactly copied from the corresponding section (gene) of the DNA molecule.

Further studies have shown that the so-called pro-mRNA is synthesized during transcription, a precursor of the mature mRNA involved in translation. Pro-mRNA is much larger and contains fragments that do not code for the synthesis of the corresponding polypeptide chain. In DNA, along with regions encoding rRNA, tRNA, and polypeptides, there are fragments that do not contain genetic information. They are called introns, in contrast to the coding fragments, which are called exons. Introns are found in many regions of DNA molecules. So, for example, in one gene - a DNA region encoding chicken ovalbumin, there are 7 introns, in the rat serum albumin gene - 13 introns. The length of the intron varies from two hundred to a thousand pairs of DNA nucleotides. Introns are read (transcribed) at the same time as exons, so pro-mRNA is significantly longer than mature mRNA. In the nucleus in pro-mRNA, introns are cut out by special enzymes, and exon fragments are “spliced” together in a strict order. This process is called splicing. In the process of splicing, a mature mRNA is formed, which contains only the information that is necessary for the synthesis of the corresponding polypeptide, that is, the informative part of the structural gene.

The meaning and functions of introns have not yet been fully elucidated, but it has been established that if only portions of exons are read in DNA, mature mRNA is not formed. The splicing process has been studied using the ovalbumin gene as an example. It contains one exon and 7 introns. First, pro-mRNA containing 7700 nucleotides is synthesized on DNA. Then, in pro-mRNA, the number of nucleotides decreases to 6800, then to 5600, 4850, 3800, 3400, etc. to 1372 nucleotides corresponding to the exon. The mRNA containing 1372 nucleotides leaves the nucleus into the cytoplasm, enters the ribosome and synthesizes the corresponding polypeptide.

The next stage of biosynthesis - translation - occurs in the cytoplasm on ribosomes with the participation of tRNA.

Transfer RNAs are synthesized in the nucleus, but function in a free state in the cytoplasm of the cell. One tRNA molecule contains 76-85 nucleotides and has a rather complex structure resembling a clover leaf. Three sections of tRNA are of particular importance: 1) an anticodon, consisting of three nucleotides, which determines the site of attachment of the tRNA to the corresponding complementary codon (mRNA) on the ribosome; 2) a site that determines the specificity of tRNA, the ability of a given molecule to attach only to a specific amino acid; 3) an acceptor site to which an amino acid is attached. It is the same for all tRNAs and consists of three nucleotides - C-C-A. The attachment of an amino acid to tRNA is preceded by its activation by the enzyme aminoacyl-tRNA synthetase. This enzyme is specific for each amino acid. The activated amino acid attaches to the corresponding tRNA and is delivered by it to the ribosome.

The central place in translation belongs to ribosomes - ribonucleoprotein organelles of the cytoplasm, which are present in many in it. The size of ribosomes in prokaryotes is on average 30x30x20 nm, in eukaryotes - 40x40x20 nm. Usually their sizes are determined in units of sedimentation (S) - the rate of sedimentation during centrifugation in the appropriate medium. In the bacterium Escherichia coli, the ribosome has a size of 70S and consists of two subparticles, one of which has a constant of 30S, the second 50S, and contains 64% ribosomal RNA and 36% protein.

The mRNA molecule exits the nucleus into the cytoplasm and attaches to a small subunit of the ribosome. Translation begins with the so-called start codon (synthesis initiator) - A-U-G-. When tRNA delivers an activated amino acid to the ribosome, its anticodon is hydrogen bonded to the nucleotides of the mRNA's complementary codon. The acceptor end of the tRNA with the corresponding amino acid is attached to the surface of the large subunit of the ribosome. After the first amino acid, another tRNA delivers the next amino acid, and thus a polypeptide chain is synthesized on the ribosome. An mRNA molecule usually works on several (5-20) ribosomes at once, connected into polysomes. The beginning of the synthesis of a polypeptide chain is called initiation, its growth is called elongation. The sequence of amino acids in a polypeptide chain is determined by the sequence of codons in mRNA. Synthesis of the polypeptide chain stops when one of the terminator codons appears on the mRNA - UAA, UAG or UGA. The end of the synthesis of a given polypeptide chain is called termination.

It has been established that in animal cells the polypeptide chain lengthens by 7 amino acids in one second, and mRNA advances on the ribosome by 21 nucleotides. In bacteria, this process proceeds two to three times faster.

Consequently, the synthesis of the primary structure of the protein molecule - the polypeptide chain - occurs on the ribosome in accordance with the order of nucleotide alternation in the matrix ribonucleic acid - mRNA. It does not depend on the structure of the ribosome.

The process of protein synthesis in a cell is called biosynthesis. It consists of two main stages - transcription and translation (Fig. 4.5). First step - transcription of genetic information- the process of synthesis of single-stranded mRNA K complementary to one sense strand of DNA, that is, the transfer of genetic information about the nucleotide structure of DNA to mRNA. Through the holes of the nuclear membrane, mRNA enters the channels of the endoplasmic reticulum and here it combines with ribosomes. Protein synthesis occurs on the mRNA molecule, and the ribosomes move along it and leave it by the end of the synthesis of the polypeptide chain (Fig. 4.6).

Figure 4.6 shows only two triplets: the complementary anticodon, corresponding to the mRNA column, and the CCA triplet, to which amino acids (LA) are attached.
Amino acids located in the cytoplasm are activated by enzymes, after which they bind to another type of RNA - transport. It will skew the amino acids to the ribosomes. Various tRNAs deliver amino acids to the ribosome and arrange them according to the sequence of mRNA triplets. Three consecutive nucleotides encoding a specific amino acid were called a codon (mRNA), and an unbreakable triplet was called an anticodon (tRNA). Codons are not separated from each other. Delivering a specific amino acid, tRNA interacts with mRNA (codon-anticodon). and the amino acid joins the growing floor and peptide chain. It is quite obvious that the synthesis of a polypeptide, that is, the arrangement of amino acids in it, is determined by the mRNA nucleotide sequence.

The second stage of biosynthesis - broadcast- translation of genetic information from mRNA into the amino acid sequence of the polypeptide chain.
In the sequence of nucleotides in the triplet, a certain amino acid is encoded. It has been established that the genetic code is triplet, that is, each amino acid is encoded by a combination of three nucleotides. If the code is a triplet, then 64 codons (4v3) can be made from four nitrogenous bases; this is more than enough to code for 20 amino acids. A new property of the genetic code has been revealed - its redundancy, that is, some amino acids encode not one, but a greater number of triplets. Of the 64 codons, three are recognized as stop codons; they cause the termination (termination) or interruption of genetic translation (Table 4.2).

The genetic code is non-overlapping. If the codons overlapped, then a change in one pair of bases would result in a change in two amino acids in the polypeptide chain, and this does not happen. In addition, it is universal - the same for the biosynthesis of proteins of living beings. The universality of the code testifies to the unity of life on Earth. Thus, the genetic code is a system for recording hereditary information in nucleic acids in the form of a sequence of nucleotides.
Subsequently, the way for the implementation of genetic information in the cell was supplemented by reverse transcription (DNA synthesis on an RNA template) - DNA and RNA replication (Fig. 4.7).

A gene is a section of DNA. encoding the primary structure of a polypeptide or nucleic acid. Several different genes are involved in the control of polypeptide chain synthesis: structural genes, regulator gene, and operator gene. The mechanism of regulation of the genetic code was discovered by French scientists F. Jacob and J. Monod in 1961 on E. coli bacteria and was called the induction-repression mechanism. Structural genes code for the sequence of amino acids in polypeptides. Usually, for structural genes, there is a common regulatory system consisting of a regulator gene and an operator gene. The regulator gene determines the synthesis of a repressor protein, which, when combined with an operator, “allows” or “prohibits” the reading of information from the corresponding structural genes. The operator gene and the structural genes following it were called an operon - a unit for reading genetic information, a transcription unit (Fig. 4.8).

For example, for the normal life of E. coli, milk sugar - lactose is necessary. It has a lactose region (lac-operon), on which three structural genes for the breakdown of lactose are located. If lactose does not enter the cell, then the repressor protein produced by the regulator gene binds to the operator and thereby “prohibits” transcription (mRNA synthesis) from the entire operon. If lactose enters the cell, then the function of the repressor protein is blocked, transcription, translation, synthesis of enzyme proteins and lactose thawing begin. After the breakdown of all lactose, the activity of the repressor protein is restored and transcription is suppressed.
Thus, genes can be on and off. Their regulation is influenced by metabolic products, hormones. The gene functions in the DNA-RNA-protein system, which is influenced by the interaction of genes and environmental factors.

From a biochemical point of view, muscle protein synthesis is a very complex process. Information about the structure of all proteins necessary for the body contains DNA, located in the nucleus of cells. The functions of a protein depend on the sequence of amino acids in their structure. And this sequence is encoded by the sequence of DNA nucleotides, in which each amino acid corresponds to a group of three nucleotides - a triplet. And each section of DNA - the genome - is responsible for the synthesis of one type of protein.

Protein is built by ribosomes in the cytoplasm. The necessary information about its structure is transmitted from the nucleus to the ribosomes with the help of mRNA (messenger RNA) - a kind of "copy" of the desired genome. i-RNA synthesis is the first step in protein biosynthesis, called transcription("rewriting").

The second stage of protein synthesis in cells is broadcast("translation" of the DNA nucleotide code into a sequence of amino acids). At this stage, the i-RNA attaches to the ribosome, then the ribosome begins to move along the i-RNA chain from the start codon and attaches the i-RNA at each codon (nucleotide triplet encoding information about one amino acid) to the i-RNA - amino acids brought by t-RNA (transfer RNA ). T-RNAs contain a specific amino acid molecule and an anticodon corresponding to a specific mRNA codon. The ribosome adds an amino acid to the growing protein chain, then detaches the tRNA and moves on to the next codon. This happens until the ribosome encounters a terminator - a stop codon. After that, the synthesis of the protein molecule stops and it detaches from the ribosome. It remains only to transport the finished protein molecule to the growing muscle cell.

Synthesis activation

The main mechanism that triggers protein synthesis in muscles is the activation of the well-known mTOR (mammalian target of rapamycin - that is, “the target of rapamycin in mammals”). It is called a “target” because mTOR is responsible for the growth and reproduction of cells, and these processes are blocked by special inhibitors (for example, rapamycin) that act specifically on this protein.

It is important for an athlete that protein synthesis and destruction are constantly taking place in the muscles, which ensure the renewal of muscle tissue. And if we want our muscles to grow, we need to make sure that over a certain period of protein synthesis exceeds its destruction. To do this, we consider the processes of activation of protein synthesis, the key element of which is mTOR.

Biochemically, mTOR is an enzyme protein (belonging to the group of protein kinases) that stimulates the translation process, i.e. protein synthesis by ribosomes into mRNA (it is also called mRNA - messenger RNA). In turn, mTOR itself is activated by amino acids (leucine, isoleucine, etc.) and growth factors (various hormones - growth hormone, insulin, etc.).

Muscle loading stimulates mTOR indirectly, through a signaling system for muscle breakdown and increased secretion of growth factors (eg, mechano growth factor).

Protein balance

So, if our task is achieve a positive protein balance , i.e. the superiority of protein synthesis over its destruction, then we should reduce catabolism (muscle breakdown) and stimulate their growth. And we have a great opportunity to succeed in this - the so-called. "protein-carbohydrate window". Everyone understands that in the period shortly from the start of training, the athlete's body experiences an acute shortage of nutrients, which lasts about one and a half to two hours after the end of training, until the body makes up for the lack of necessary substances from its own resources. Given that the rate of absorption and assimilation of amino acids in a protein shake is an hour and a half, we get the limits of the protein-carbohydrate window, the intake of amino acids and carbohydrates in which has a high absorption efficiency - from 1.5 hours before training to 1.5 hours after.

According to the wisdom of Nature, many substances (such as) have the ability not only to stimulate protein synthesis, but also to suppress its destruction (for example, they inhibit the action of cortisol). It is believed that taking protein (preferably in the form of or even, for example) and carbohydrates can give a good anabolic effect in any of the three periods of the protein-carbohydrate window - before training, during training and after training. But it is strongly recommended to take BCAAs immediately before or immediately after a workout, as well as taking carbohydrates with a high glycemic index during a workout, and be sure to take protein within an hour after a workout. So you will provide your body with all the necessary substances for active protein synthesis.

Protein biosynthesis takes place in every living cell. It is most active in young growing cells, where proteins are synthesized for the construction of their organelles, as well as in secretory cells, where enzyme proteins and hormone proteins are synthesized.

The main role in determining the structure of proteins belongs to DNA. A piece of DNA containing information about the structure of a single protein is called a gene. A DNA molecule contains several hundred genes. A DNA molecule contains a code for the sequence of amino acids in a protein in the form of definitely combined nucleotides. The DNA code has been deciphered almost completely. Its essence is as follows. Each amino acid corresponds to a section of the DNA chain of three adjacent nucleotides.

For example, the T-T-T section corresponds to the amino acid lysine, the A-C-A segment corresponds to cystine, C-A-A to valine, etc. There are 20 different amino acids, the number of possible combinations of 4 nucleotides by 3 is 64. Therefore , there are more than enough triplets to encode all amino acids.

Protein synthesis is a complex multi-stage process representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.

Since DNA is located in the cell nucleus, and protein synthesis occurs in the cytoplasm, there is an intermediary that transmits information from DNA to ribosomes. Such an intermediary is mRNA. :

In protein biosynthesis, the following stages are determined, which take place in different parts of the cell:

The first stage - the synthesis of i-RNA occurs in the nucleus, during which the information contained in the DNA gene is rewritten into i-RNA. This process is called transcription (from the Latin "transcript" - rewriting).
At the second stage, amino acids are connected to t-RNA molecules, which sequentially consist of three nucleotides - anticodons, with the help of which its triplet codon is determined.
The third stage is the process of direct synthesis of polypeptide bonds, called translation. It occurs in ribosomes.
At the fourth stage, the formation of the secondary and tertiary structure of the protein occurs, that is, the formation of the final structure of the protein.

Thus, in the process of protein biosynthesis, new protein molecules are formed in accordance with the exact information embedded in DNA. This process ensures the renewal of proteins, metabolic processes, growth and development of cells, that is, all the processes of cell vital activity.

Chromosomes (from the Greek "chroma" - color, "soma" - body) are very important structures of the cell nucleus. They play a major role in the process of cell division, ensuring the transfer of hereditary information from one generation to another. They are thin strands of DNA attached to proteins. The filaments are called chromatids and are made up of DNA, basic proteins (histones), and acidic proteins.

In a non-dividing cell, the chromosomes fill the entire volume of the nucleus and are not visible under a microscope. Before division begins, DNA spiralization occurs and each chromosome becomes visible under a microscope. During spiralization, chromosomes are reduced tens of thousands of times. In this state, the chromosomes look like two identical threads (chromatids) lying side by side, connected by a common site - the centromere.

Each organism is characterized by a constant number and structure of chromosomes. In somatic cells, the chromosomes are always paired, that is, in the nucleus there are two identical chromosomes that make up one pair. Such chromosomes are called homologous, and paired sets of chromosomes in somatic cells are called diploid.

So, the diploid set of chromosomes in humans consists of 46 chromosomes, forming 23 pairs. Each pair consists of two identical (homologous) chromosomes.

Structural features of chromosomes make it possible to distinguish their 7 groups, which are denoted by the Latin letters A, B, C, D, E, F, G. All pairs of chromosomes have serial numbers.

Men and women have 22 pairs of identical chromosomes. They are called autosomes. Men and women differ in one pair of chromosomes, which are called sex chromosomes. They are designated by letters - large X (group C) and small Y (group C,). The female body has 22 pairs of autosomes and one pair (XX) of sex chromosomes. Males have 22 pairs of autosomes and one pair (XY) of sex chromosomes.

Unlike somatic cells, germ cells contain half the set of chromosomes, that is, they contain one chromosome of each pair! Such a set is called haploid. The haploid set of chromosomes arises in the process of cell maturation.

Biosynthesis of proteins goes in every living cell. It is most active in young growing cells, where proteins are synthesized for the construction of their organelles, as well as in secretory cells, where enzyme proteins and hormone proteins are synthesized.

The main role in determining the structure of proteins belongs to DNA. A piece of DNA containing information about the structure of a single protein is called genome. A DNA molecule contains several hundred genes. A DNA molecule contains a code for the sequence of amino acids in a protein in the form of definitely combined nucleotides. The DNA code has been deciphered almost completely. Its essence is as follows. Each amino acid corresponds to a section of the DNA chain of three adjacent nucleotides.

protein synthesis - a complex multi-stage process, representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.

Since DNA is located in the cell nucleus, and protein synthesis occurs in the cytoplasm, there is an intermediary that transmits information from DNA to ribosomes. Such an intermediary is mRNA.

In protein biosynthesis, the following stages are determined, which take place in different parts of the cell:

The first stage - the synthesis of i-RNA occurs in the nucleus, during which the information contained in the DNA gene is rewritten into i-RNA. This process is called transcription(from the Latin "transcript" - rewriting).
At the second stage, amino acids are combined with t-RNA molecules, which consist of three nucleotides in sequence - anticodonov, with the help of which its triplet codon is determined.
The third stage is the process of direct synthesis of polypeptide bonds, called broadcast. It occurs in ribosomes.
At the fourth stage, the formation of the secondary and tertiary structure of the protein occurs, that is, formation of the final protein structure.

Synthesis of messenger RNA (i-RNA) occurs in the nucleus. It is carried out along one of the DNA strands with the help of enzymes and taking into account the principle of complementarity of nitrogenous bases. The process of rewriting the information contained in the DNA genes to the synthesized mRNA molecule is called transcription . Obviously, the information is rewritten in the form of a sequence of RNA nucleotides. The DNA strand in this case acts as a template. In the RNA molecule, in the process of its formation, instead of the nitrogenous base - thymine, uration is included.

G - C - A - A - C - T - a fragment of one of the chains of the DNA molecule; C - G - U - U - G - A - a fragment of the messenger RNA molecule.

RNA molecules are individual, each of them carries information about one gene. Next, the mRNA molecules leave the cell nucleus through the pores of the nuclear envelope and are directed to the cytoplasm to the ribosomes. Amino acids are also delivered here with the help of transport RNA (t-RNA). The tRNA molecule consists of 70–80 nucleotides. The general appearance of the molecule resembles a clover leaf.

At the top of the sheet is anticodon(coding triplet of nucleotides), which corresponds to a specific amino acid. Therefore, each amino acid has its own specific t-RNA. The process of assembling a protein molecule takes place in ribosomes and is called broadcast. Several ribosomes are sequentially located on one mRNA molecule. Two mRNA triplets can fit in the functional center of each ribosome. The code triplet of nucleotides - a t-RNA molecule that has approached the site of protein synthesis, corresponds to the triplet of nucleotides of an mRNA that is currently in the functional center of the ribosome. Then the ribosome along the mRNA chain makes a step equal to three nucleotides. The amino acid is separated from the tRNA and becomes a chain of protein monomers. The released tRNA goes aside and after a while can reconnect with a certain acid, which will be transported to the site. protein synthesis. Thus, the sequence of nucleotides in the DNA triplet corresponds to the sequence of nucleotides in the mRNA triplet.

In the most complex process of protein biosynthesis, the functions of many substances and organelles of the cell are realized.