Proteins

Proteins

Proteins are compounds consisting of one or more polypeptides that generally assist in some biological function. Proteins assume the most thermodynamically stable form in their environment of residence, for example, in the reducing, aqueous environment inside the cell, disulfide bonds are reduced to thiols, whereas in the extracellular region, the disulfide bonds remain oxidized. Reversible covalent reactions, such as disulfide reduction or exchanges, can have enormous implications on a protein’s tertiary or quaternary structure, and these reversible covalent reactions are often utilized by living organisms as a method of protein activation or regulation [1].

Non-covalent bonds, along with reversible covalent bonds, also play a pivotal role in determining the structure and function of a protein. Proteins have been described as a thermodynamic compromise; the “thermodynamic compromise” of protein’s structure occurs as the polypeptide attempts to fold in such a fashion so as to maximize favourable non-covalent bond interactions [1]. In order to do this, proteins will by themselves or with the assistance of chaperonins place most of the polar, hydrophilic amino acid residues into contact with the aqueous cytoplasm or extracellular environment, while directing most of the non-polar, hydrophobic residues to the interior of the protein or to the areas that will interact with the lipid tails of the plasma membrane bilayer [1].

Figure 1 : A transmembrane protein (purple) that spans the lipid bilayer (yellow and gray). The hydrophobic portions of the protein and lipid bilayer are present on the inside, away from the aqueous environment. The structure of the protein dictates the function it will perform. The protein's structure is driven by many things, including the environment that surrounds the protein, the specific secondary structure elements, the position of the secondary structure elements, and the amino acid residues that comprises the primary structure [1] .T he tertiary and the quaternary structure that the protein adopts eventually determines the substrate that the protein will bind, and how and where other proteins or small molecules may interact with it [1]. T herefore, the size, shape, and residue composition of the surface of a protein governs as to how it will interact with both substrates and other proteins, through complementarity of shape, charge, and physical characteristics [1].

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