Protein structure

Proteins are amino acid chains, made up from 20 different L-α-amino acids, also referred to as residues, that fold into unique three-dimensional protein structures. The shape into a which a protein naturally folds is known as its native state, which is determined by its sequence of amino acids. Below about 40 residues the term peptide is frequently used. A certain number of residues is necessary to perform a particular biochemical function, and around 40-50 residues appears to be the lower limit for a functional domain size. Protein sizes range from this lower limit to several thousand residues in multi-functional proteins. However, the current estimate for the average protein length is around 300 residues. Very large aggregates can be formed from protein subunits, for example many thousand actin molecules assemble into a an actin filament. Large protein complexes with RNA are found in the ribosome particles, which are in fact 'ribozymes'.

Related Topics:
Proteins - Amino acid - Fold - Native state - Peptide - Biochemical - Domain - Protein subunit - Actin - RNA - Ribozyme

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Biochemists refer to four distinct aspects of a protein's structure:

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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 of 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 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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Protein structure: Amino acid structure ►