Similarity of different β ‐strands flanked in loops by glycines and prolines from distinct (α/β) 8 ‐barrel enzymes: Chance or a homology?

Many (α/β) 8 ‐barrel enzymes contain their conserved sequence regions at or around the β‐strand segments that are often preceded and succeeded by glycines and prolines, respectively. α‐Amylase is one of these enzymes. Its sequences exhibit a very low degree of similarity, but strong conservation is seen around its β‐strands. These conserved regions were used in the search for similarities with β‐strands of other (α/β) 8 ‐barrel enzymes. The analysis revealed an interesting similarity between the segment around the β2‐strand of α‐amylase and the one around the β4‐strand of glycolate oxidase that are flanked in loops by glycines and prolines. The similarity can be further extended on other members of the α‐amylase and glycolate oxidase subfamilies, i.e., cyclodextrin glycosyltransferase and oligo‐l,6‐glucosidase, and flavocytochrome b 2 , respectively. Moreover, the α‐subunit of tryptophan synthase, the (α/β) 8 ‐barrel enzyme belonging to the other subfamily of (α/β) 8 ‐barrels, has both investigated strands, β2 and β4, similar to β2 of α‐amylase and β4 of glycolate oxidase. The possibilities of whether this similarity exists only by chance or is a consequence of some processes during the evolution of (α/β) 8 ‐barrel proteins are briefly discussed.