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Proteins are the chief actors within the cell, said to be carrying out the duties specified by the information encoded in genes.[9] With the exception of certain types of RNA,[link widoczny dla zalogowanych], most other biological molecules are relatively inert elements upon which proteins act. Proteins make up half the dry weight of an Escherichia colicell,[link widoczny dla zalogowanych] whereas other macromolecules such as DNA and RNA make up only 3% and 20%, respectively.[26] The set of proteins expressed in a particular cell or cell type is known as its proteome.
The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly. The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or "pocket" on the molecular surface. This binding ability is mediated by the tertiary structure of the protein, which defines the binding site pocket, and by the chemical properties of the surrounding amino acids' side chains. Protein binding can be extraordinarily tight and specific; for example, the ribonuclease inhibitor protein binds to human angiogenin with a sub-femtomolardissociation constant (<10?15 M) but does not bind at all to its amphibian homolog onconase (>1 M). Extremely minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding;[link widoczny dla zalogowanych] for example, the aminoacyl tRNA synthetase specific to the amino acid valinediscriminates against the very similar side chain of the amino acid isoleucine.[27]
Proteins can bind to other proteins as well as to small-molecule substrates. When proteins bind specifically to other copies of the same molecule, they can oligomerize to form fibrils; this process occurs often in structural proteins that consist of globular monomers that self-associate to form rigid fibers. Protein–protein interactions also regulate enzymatic activity, control progression through the cell cycle, and allow the assembly of large protein complexes that carry out many closely related reactions with a common biological function. Proteins can also bind to, or even be integrated into, cell membranes. The ability of binding partners to induce conformational changes in proteins allows the construction of enormously complex signalingnetworks.[28] Importantly, as interactions between proteins are reversible,[link widoczny dla zalogowanych] and depend heavily on the availability of different groups of partner proteins to form aggregates that are capable to carry out discrete sets of function, study of the interactions between specific proteins is a key to understand important aspects of cellular function, and ultimately the properties that distinguish particular cell types.