proteins (rus. белки otherwise полипептиды) — highmolecular weight natural polymers built from amino acid residues joined by amide (peptide) bond –CO–NH–.

Description

Each protein is characterised by a specific amino acid sequence and an individual spatial structure (conformation). There are 4 main categories of proteins:

- structural (forming cell structures);

- enzymes (catalysing chemical reactions);

- regulatory (controlling gene expression or activity of other proteins);

- transport (carrying other molecules inside the cell or through the cell membrane).

Functioning of proteins underlies the most important processes of vital activity. Simple proteins consist only of amino acid residues, while complex proteins may include metal ions (metalloproteins) or pigment (chromoproteins), form stable complexes with lipids (lipoproteins) and nucleic acids (nucleoproteins), or contain covalently bound phosphoric acid (phosphoproteins) or carbohydrate (glycoproteins) residues.

Protein biosynthesis is carried out through the process of translation in subcellular particles – ribosomes (ribonucleoprotein complexes). The primary structure of proteins (amino acid sequence) is determined by the nucleotide sequence in the corresponding genes. In the process of transcription this information is transferred to the matrix RNA by an enzyme called DNA-dependent RNA polymerase. Matrix RNA combines with the ribosome and serves as a template for protein synthesis. The synthesised polypeptide chains emerge from the ribosome fold spontaneously into the protein-specific three-dimensional conformation, and are modified through reactions of various functional groups of amino acid residues and cleavage of peptide bonds.

The three-dimensional structure of the protein plays an important role in its specific functional activity. It is determined by the interaction of lower level structures. There are four levels of protein structure. The primary structure is the sequence of amino acids in the polypeptide chain, which determines the subsequent conformational event. The secondary structure refers to the polypeptide chain folding into secondary structure elements (alpha-helical regions, beta-structural layers, etc.) through formation of hydrogen bonds. The tertiary structure refers to the spatial arrangement of secondary structure elements stabilised by different types of interactions (covalent, ionic and hydrophilic-hydrophobic). The quaternary structure refers to configuration and interaction between individual polypeptide chains and describes spatial organisation of polymeric proteins (i.e. consisting of two or more polypeptide chains). Protein chains that make up proteins with a quaternary structure are formed on the ribosomes separately, and only after their synthesis is complete do they form a supramolecular structure (or a complex molecule, if disulphide bridges are formed between different polypeptide chains). Proteins with a quaternary structure may contain both identical and different polypeptide chains. The quaternary structure is stabilised by the same interactions as the tertiary structure.

20 standard amino acids are commonly used to synthetize proteins. Their multiple combinations provide a great variety of proteins with different properties. Often, in living organisms several protein molecules form complex functional units with each other and with other molecules, such as photosynthetic pigment-protein complex. Supramolecular protein complexes may consist of tens of molecules; many of them are comparable in size to ribosomes, and in recent years such complexes are often described as organelles (e.g., the proteasome – protein complex that degrades proteins at the end of their life cycle).

In-depth understanding of the protein structure has enabled the following additional levels of its organisation to be distinguished: the supersecondary structure (ensembles of interacting secondary structures such as supercoiling alpha-helices – two alpha-helices twisting around each other), structural domains (corresponding to the globule with the diameter of 2.5 nm according to electron density maps to meet the principle of simplicity of protein folding), globular proteins, aggregates.

Proteins are one of the main functional structures of all living things. These structures are widely used in nanobiotechnology and nanomedicine. They include: molecular vectors for targeted drug delivery (antibodies), chemical sensors (enzymes and ion channels), biogenic and universal nanoparticles for therapeutic or diagnostic applications, and more.

Illustrations

Protein structure levels: 1 — primary; 2 — secondary; 3 — tertiary; 4 — quaternary.
Protein structure levels: 1 — primary; 2 — secondary; 3 — tertiary; 4 — quaternary.

Authors

  • Kurochkin Ilya N.
  • Naroditsky Boris S.
  • Nesterenko Lyudmila N.

Sources

  1. Clark, D.P. and Russell, L.D. 2010. Molecular Biology Made Simple and Fun. Cache River Press/Quick Publishing, St. Louis, MO. 4th edition.
  2. B. Glick, J. Pasternak. Molecular Biotechnology: Principles and Applications of Recombinant DNA. — 3rd ed. Sigma Publishing, 2003, 784 pp.
  3. Protein // Wikipedia, the free encyclopedia. — http://en.wikipedia.org/wiki/Protein (reference date: 12.12.2011).
  4. Proteins //Chemical encyclopedia (in Russian) V.1 — Moscow: The Great Soviet Encyclopedia, 1998. P. 247–254.

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