The basic structure of proteins

The building blocks of proteins are amino acids, bound together in a linear fashion by covalent peptide bonds. Each protein has a precise length and amino acid sequence dictated or transcribed by messenger RNA that in turn is translated from the DNA. Once made the protein may be modified, or chopped into smaller pieces, or carbohydrate, lipid or phosphate moieties may be added by the action of enzymes within a plant or animal cell. The linear sequence of amino acids is termed the primary structure of the protein. Proteins are rarely linear in the native form but form distinctive three-dimensional structures. This is due to chemical interactions between amino acids in close proximity, causing the chain to form twists that force it into spirals, termed alpha-helixes, and sharp bends resulting in so-called beta-pleated sheets. These basic forms are part of the secondary structure of the protein. The arrangement of these secondary structures in relation to each other gives rise to the tertiary structure. This results from non-covalent interactions between the different regions of the same protein or polypeptide molecule. In addition many functional proteins consist of aggregates of two or more polypeptide chains, that are homogeneous or heterogeneous. This is termed the quaternary structure. The three-dimensional shape and chemical nature of the amino acid backbone and additional groups contribute to the functional and antibody-binding properties of the molecule.

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