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'.

Amino acid structure

The basic structure of an α-amino acid is quite simple. R denotes any one of the 20 possible side chains (see table below). We notice that the Cα-atom has 4 different ligands (the H is omitted in the drawing) and is thus chiral. An easy trick to remember the correct L-form is the CORN-rule: when the Cα-atom is viewed with the H in front, the residues read "CO-R-N" in a clockwise direction. 

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The different side chains R determine the chemical properties of the amino acid or residue (the residue is the amino acid side chain plus the peptide backbone, see below).

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