Serine protease

A serine protease uses a serine side chain as a catalytic nucleophile in peptide-bond hydrolysis. In the chymotrypsin family, a catalytic arrangement involving serine, histidine and aspartate promotes formation and hydrolysis of a covalent acyl-enzyme intermediate. The catalytic mechanism and substrate-recognition pocket are distinct: related serine proteases can prefer different cleavage sites. ^[ref]

Factor Xa is the activated serine protease derived from Factor X by limited proteolysis. It catalyses cleavage of prothrombin to generate thrombin, an important activation step in coagulation. Efficient conversion is organized by factor Va, calcium and phospholipids.^[ref]

Purified-component experiments varied the phospholipid surface and the presence of factor Va, showing that apparent kinetic parameters depend on how the components assemble. The same amount of factor Xa can therefore produce different reaction rates in different surroundings.^[ref]

For an invented catalytic accounting example, 1 nmol of active factor Xa generates 100 nmol of activated product during an interval. That is 100 product molecules per initial enzyme molecule, demonstrating catalytic reuse rather than one-to-one consumption. The value is not a measured turnover number: a true rate constant requires a specified time, substrate availability and activation pathway.

A family of serine proteases with a fold distinct from the chymotrypsin family. Its Asp–His–Ser sequence order contrasts with His–Asp–Ser in chymotrypsin-like enzymes despite similar local catalytic chemistry.

A protease whose chymotrypsin-family examples favour small substrate side chains through an obstructed specificity pocket. Related fold and catalytic architecture do not imply identical specificity.

Chymotrypsin is a digestive serine protease that preferentially cleaves peptide bonds after certain bulky hydrophobic residues, especially aromatic residues. Its binding pocket favours those side chains, while its catalytic machinery hydrolyses the adjacent peptide bond. It is produced as an inactive precursor, limiting unwanted digestion before activation. Sequence preference is conditional rather than an assurance that every matching site in a folded protein is equally accessible. ^[ref]