Protein

A protein (in Greek πρωτεϊνη = first thread) is a complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds. Proteins are essential to the structure and function of all living cells and viruses.

Structure

Main article: Protein structure

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Proteins are amino acid chains that fold into unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native state, which is determined by its sequence of amino acids. Biochemists refer to four distinct aspects of a protein's structure:

Related Topics:
Fold - Native state

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• Primary structure: the amino acid sequence
• Secondary structure: highly patterned sub-structures—alpha helix and beta sheet—or segments of chain that assume no stable shape. Secondary structures are locally defined, meaning that there can be many different secondary motifs present in one single protein molecule.
• Tertiary structure: the overall shape of a single protein molecule; the spatial relationship of the secondary structural motifs to one another
• Quaternary structure: the shape or structure that results from the union of more than one protein molecule, usually called subunit proteins subunits in this context, which function as part of the larger assembly or protein complex.

In addition to these levels of structure, proteins may shift between several similar structures in performing their biological function. In the context of these functional rearrangements, these tertiary or quaternary structures are usually referred to as "conformations," and transitions between them are called conformational changes.

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The primary structure is held together by covalent peptide bonds, which are made during the process of translation. The secondary structures are held together by hydrogen bonds. The tertiary structure is held together primarily by hydrophobic interactions but hydrogen bonds, ionic interactions, and disulfide bonds are usually involved too.

Related Topics:
Covalent - Peptide bond - Translation - Hydrogen bond - Hydrophobic - Disulfide bond

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The process by which the higher structures form is called protein folding and is a consequence of the primary structure. The mechanism of protein folding is not entirely understood. Although any unique polypeptide may have more than one stable folded conformation, each conformation has its own biological activity and only one conformation is considered to be the active, or native conformation.

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The two ends of the amino acid chain are referred to as the carboxy terminus (C-terminus) and the amino terminus (N-terminus) based on the nature of the free group on each extremity.

Related Topics:
Carboxy terminus - Amino terminus

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