Protease

A protease catalyses hydrolysis of peptide bonds in proteins or peptides. Different proteases recognize different sequences or structures, and some cut within a chain while others remove terminal residues. Digestion, signalling activation and controlled protein degradation all use this chemistry. The sequence specificity of chymotrypsin, for example, differs from indiscriminate breakdown of every peptide bond. ^[ref]

An aspartic protease is a protease whose catalytic chemistry depends on aspartic acid side chains. In common examples, their carboxyl groups position and activate a water molecule for peptide bond hydrolysis. Water supplies the attacking nucleophile, unlike a mechanism in which an enzyme side chain itself makes the initial covalent attack.^[ref]

The name concerns catalytic groups, rather than requiring an aspartic-acid residue at every substrate cleavage site. Substrate recognition and catalytic mechanism are distinct properties.

HIV-1 protease provides an experimentally characterized example. In 1989, T. D. Meek and colleagues measured apparent masses of approximately 11,000 under denaturing conditions and 22,000 under nondenaturing conditions; protein cross-linking supplied additional evidence for an active protein dimer. The ratio $22,000/11,000=2$ explains the subunit interpretation.^[ref]

The same study found pH-dependent inactivation consistent with a required unprotonated carboxyl group in the active site. Combining this chemical perturbation with cleavage activity and subunit measurements supported a catalytic classification more specifically than the enzyme's molecular mass alone.^[ref]

A cytotoxic-lymphocyte protease acting in target cells to promote death. Directional secretion at an immune synapse helps restrict its action to recognized targets.

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]