User:JamesMilnerWhite/sandbox

The Beta link is a motif in proteins and polypeptides consisting of 5 amino acid residues. It is characterized by an overlap of a G1 type beta bulge and a type II beta turn, in which the 3rd and 4th amino acids of the beta-turn are also the bulged-out side of the beta bulge

G1 beta bulges (beta bulges with a glycine as the first residue of the bulged-out side) typically occur in one of two composite situations, either within a beta bulge loop, or as a component of a beta link[2]. The latter commonly occur at the corners or ends of antiparallel beta sheets in proteins and polypeptides. The beta turn is situated perpendicular to the beta-sheet such that the beta link often acts as a join between the beta-sheet and another secondary structure element; sometimes the beta link connects opposite strands at the end of a single beta-barrel.

In serine proteases the eponymous serine is situated within a well-conserved beta link. The serine occurs at residue 2 of the type II beta-turn (with the oxyanion hole or nest (protein structural motif) at residues 0, 1 and 2). The beta link may provide a hydrogen bonded framework to assist the serine to adopt the correct conformation for catalysis. Some “serine proteases” have a cysteine in place of the serine, performing the same catalytic function, and the beta link is conserved in these proteins too.