3.2.2. Post-translational modification of proteins

In majority of cases proteins are synthesized in the form of precursors which are biologically inactive molecules. Their functional activity is expressed due to several conversions which are called post-synthetic or post-translational modification (processing).

Examples of post-translational modification of proteins are:

1. Proteolytic cleavage of N-terminal formyl-methionine or methionine.

2. Cleavage of signal peptides.

3. Partial proteolysis.

4. Post-translational modification of proteins on amino acid radical: covalent addition of prosthetic group, methylation of lysine and arginine radicals, acetylation of N-terminal amino acid, phosphorylation of histones and non-histon chromatine proteins, hydroxylation of proline radical, addition of oligosaccharide fragments (glycosylation) to asparagine, serine and threonine radicals, etc.

The formation of the correct proteins structure is done with the help of chaperones. Hydrophobic parts at the surface of the chaperones-70 globule interact with hydrophobic parts of the synthesized chain protecting it from wrong interactions with other proteins in the cytosol. Chaperones-60 take part in the correction of spatial structure of wrongly folded or damaged chain.

Mutations in the chaperone, which is the part of the eye lens lead to the opacity of the lens due to protein aggregation and the development of cataracts.

Transport of synthesized proteins through membranes

Synthesized protein comes to cytosol from the ribosome. If it is not synthesized to satisfy the needs the cell itself and refers to exported or secreted proteins then it is transported through cell membrane with the help of low molecular peptides (15-30 amino acid residues), which have hydrophobic radicals. These are signal peptides. There is a canal formed in the membrane through which signal peptide could come inside the cistern of endoplasmic reticulum. It takes molecule of the synthesized protein together with it. Under the action of signal peptidase N-terminal signal seqence is cleaved and the protein goes outside the cell through Goldgi apparatus in the form of secretory vesicles.

3.2.3. Regulation of protein biosynthesis

Concentration of many proteins is not stable in the cell. It changes according to the cell state and external environment. It is a result of rate regulation of protein synthesis and degradation.

In the cells of mammals there are 2 types of protein biosynthesis regulation:

- short-term regulation which provides organism adaptation to environmental changes;

- long-term or stable, which determines differentiation of cells and different protein composition of organs and tissues.

Regulation on transcription level (formation of the initial transcript) is one of the widely spread mechanisms of protein synthesis regulation.

There are 2 forms of regulation. They are synthesis induction (positive regulation) and synthesis repression (negative regulation).

The concepts induction and repression imply the change of protein synthesis rate in relation to the initial (basal) level. The synthesis in basal state is called constitutive synthesis.

If the rate of protein constitutive synthesis is high then the protein is regulated on the mechanism of synthesis repression and, visa versa, at low basal rate synthesis induction is used.

There are operons in the cell genetic apparatus. Operons are parts of DNA which have structural genes of particular proteins (cistrones) and regulatory sites.

Reading of the genetic code starts with a promoter which is located near the gene-operator. Gene-operator is located at terminal end of the structural gene. It can either prohibit or allow the replication of mRNA on DNA.

Activity of operon is controlled by gene-regulator. Protein-repressor provides the connection between operon and gene-regulator. Repressor is formed in ribosomal nucleus on mRNA which is synthesized on gene-regulator. It forms a complex with gene-operator and blocks mRNA synthesis and, thus, it blocks the synthesis of protein. Repressor could bind with low molecular substances: inductors or effectors. After that it loses ability to bind with gene-operator. Gene-operator goes out of gene-regulator control and mRNA synthesis starts. This is synthesis induction.

Fig. 6. Protein synthesis regulation by induction.

Fig. 7. Protein synthesis regulation by repression.

There is also the enzyme repression effect when the concentration of enzymes is lowered in cells at the increase of end products of the chain of synthesis reactions. In this case the repressor is synthesized in inactive form. It acquires the ability to repress gene-operator activity after the complex formation with synthesis product (co-repressor).

Eukaryotes mainly have positive regulatory mechanisms. The main regulatory point is the stage of transcription initiation. Regulatory elements which stimulate transcription are called enhancers and regulatory elements which repress transcription are called silencers. They could selectively bind to protein-regulators: enhancers could bind with protein-inductors and silencers could bind with protein-repressors. This regulatory elements binding with regulator proteins depends on signal molecules which are hormones, metabolites, etc.

The knowledge of ribosome structure and function of prokaryotes and eukaryotes allowed creating new types of antibiotics. Nucleic acid and protein synthesis is a key process which is necessary to keep cell life. If this mechanism is switched off then the cell will die. There are some medicines which damage the synthesis of purine bases and amino acids, nucleic acid synthesis and protein synthesis on different stages only for bacterium cells.