Papain enzyme-like activity

An interesting Ca " -activated intracellular protease, sometimes called calpain, was discovered during the last decade.The ending -pain refers to its relation with other proteolytic enzymes like papain. It may seem dangerous to have a proteolytic enzyme loose inside a cell, and it must have rather specialized functions and be under strict control. The complete primary structure of the calcium protease (Mr == 80,000) in chicken tissues has recently been deduced from the nucleotide sequence of cloned DNA. findings are quite unexpected. 

The detrimental effect of bleach should not be underestimated. Some enzymes like papain exhibit several valuable properties for laundering good thermal stability, low substrate specificity. In addition to its detrimental requirement of a low pH for maximum activity, papain contains cysteine residues, which must remain in the thiol form. In the presence of sodium perborate, they are oxidized and the enzyme activity is lost [15]. 

Cathepsin K (Cat K) is a member of the CA1 family of lysosomal cysteine proteases. This family is comprised of 11 human members (cathepsins B, C, F, H, K, L, O, S, V, W, Z) which share a common papain-like structural fold and a conserved active site Cys-Asn-His triad of residues [1-3]. These enzymes are synthesized as pre-pro-enzymes and are converted from the catalytically inactive zymogen into the active form in acidic lysosomal environment. In some cases, cathepsins are also secreted in the active form from cells. The sequence identity of... 

During the last ten years, it has become apparent that calcium-dependent papain-like peptidases called calpains (EC 3.4.22.17) represent an important intracellular nonlysosomal enzyme system [35][36], These enzymes show limited proteolytic activity at neutral pH and are present in virtually every eukaryotic cell type. They have been found to function in specific proteolytic events that alter intracellular metabolism and structure, rather than in general turnover of intracellular proteins. Calpains are composed of two nonidentical subunits, each of which contains functional calcium-binding sites. Two types of calpains, i.e., /i-calpain and m-calpain (formerly calpain I and calpain II, respectively), have been identified that differ in their Ca2+ requirement for activation. The activity of calpains is regulated by intracellular Ca2+ levels. At elevated cytoplasmic calcium concentrations, the precursor procal-pain associates with the inner surface of the cell membrane. This interaction seems to trigger autoproteolysis of procalpain, and active calpain is released into the cytoplasm [37]. 

The cysteinyl proteases include papain calpains I and II cathepsins , H, and L proline endopeptidase and interleukin-converting enzyme (ICE) and its homologs. The most well-studied cysteinyl protease is likely papain, and the first x-ray crystallographic structures of papain [193] and a peptide chloromethylketone inhibitor-papain complex [194] provided the first high resolution molecular maps of the active site. Pioneering studies in the discovery of papain substrate peptide-based inhibitors having P, electrophilic moieties such as aldehydes [195], ketones (e.g., fluoromethylketone, which has been determined [196] to exhibit selectivity for cysteinyl proteases versus serinyl proteases), semicarbazones, and nitriles are noteworthy since 13C-NMR spectro-... 

The cystatins, which are a superfamily of proteins that inhibit papain-like cysteine proteases, are a classic example of these inhibitors. The cystatins (Fig. 3) insert a wedge-hke face of the inhibitor that consists of the protein N-terminus and two hairpin loops into the V-shaped active site of a cysteine protease. The N-terminal residues bind in the S3-S1 pockets in a substrate-like manner, but the peptide then turns away from the catalytic residues and out of the active site. The two hairpin loops bind to the prime side of the active site, which provides most of the binding energy for the interaction. Thus, both the prime and the nonprime sides of the active site are occupied, but no interactions are actually made with the catalytic machinery of the enzyme (23). 

To monitor protease activities in plant extracts, Wang etal.li8 generated biotinylated peptides that contained a /3-lactone reactive group. The probes labeled several enzymes in leaf proteomes of Arabidopsis thaliana. Interestingly, these studies led to the identification of a papain-like protease called RD21 that has the unexpected ability to ligate donor molecules such as peptides or lactones, probably through a thioester intermediate, to unmodified N termini of acceptor molecules. 

Aside from calmodulin-mediated enzymes, the only other Ca " -requiring intracellular enzymatic activity that has been at all well studied is that of calpain. The calpains (EC 3.4.22.17) are intracellular cysteine proteinases characterized by millimolar (calpain I) or micromolar (calpain II) Ca " ion requirements for maximal activity. Each isozyme consists of two subunits, a 80 kDa catalytic subunit and a 30 kDa subunit. The 80 kDa subunit consists of four structural domains (I-IV). Domain II is a papain-like... 

Pepsin, pepsin-like enzymes, chymosin, rennin, and other acid proteinases have an activity optimum at pH 2.0-3.5 papain, trypsin, chymotrypsin, and similar enzymes are most active at neutral pH (pH 6-8). Subtilisin BPN, pancreatic elastase, leucine... 

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