Papain, role

The class of proton transfer (PT) reactions plays a major role in many biological processes, including various enzymatic reactions. This class of reactions will be served here as a general example and an introduction for more complicated reactions. As a specific demonstration let s consider a proton transfer between Cys 25 and His 159 in papain. This reaction can be formally described as... 

S. D. Lewis, F. A. Johnson, J. A. Shafer, Effect of Cysteine-25 on the Ionization of His-tidine-159 in Papain as Determined by Proton Nuclear Magnetic Resonance Spectroscopy. Evidence for a Hisl59-Cys25 Ion Pair and Its Possible Role in Catalysis , Biochemistry 1981, 20, 48-51. 

Cathepsin K, a member of the papain superfamily of cysteine proteinases, is selectively and highly expressed in osteoclasts (Drake et al., 1996 Bromme and Okamoto, 1995). It is secreted as a 314 amino acid proenzyme containing a 99 amino acid leader sequence (Bossard et al., 1996). Cathepsin K plays an important role in bone resorption and is a potential therapeutic target for the treatment of diseases involving excessive bone loss such as osteoporosis (Veberef al., 1997). 

In an attempt to separate the domains from the cores, we used limited degradation with several proteases. CBH I (65 kda) and CBH II (58 kda) under native conditions could only be cleaved successfully with papain (15). The cores (56 and 45 kda) and terminal peptides (11 and 13 kda) were isolated by affinity chromatography (15,16) and the scission points were determined unequivocally. The effect on the activity of these enzymes was quite remarkable (Fig. 7). The cores remained perfectly active towards soluble substrates such as those described above. They exhibited, however, a considerably decreased activity towards native (microcrystalline) cellulose. These effects could be attributed to the loss of the terminal peptides, which were recognized as binding domains, whose role is to raise the relative concentration of the intact enzymes on the cellulose surface. This aspect is discussed further below. The tertiary structures of the intact CBH I and its core in solution were examined by small angle X-ray scattering (SAXS) analysis (17,18). The molecular parameters derived for the core (Rj = 2.09 mm, Dmax = 6.5 nm) and for the intact CBH I (R = 4.27 nm, Dmax = 18 nm) indicated very different shapes for both enzymes. Models constructed on the basis of these SAXS measurements showed a tadpole structure for the intact enzyme and an isotropic ellipsoid for the core (Fig. 8). The extended, flexible tail part of the tadpole should thus be identified with the C-terminal peptide of CBH I. 

The Ca2+-dependent neutral proteases called calpains are found within the cells of higher animals. The 705-residue multidomain peptide chain of a chicken calpain contains a papain-like domain as well as a calmodulin-like domain.328 It presumably arose from fusion of the genes of these proteins. At least six calpains with similar properties are known.329 Some have a preference for myofibrillar proteins or neurofilaments.330 They presumably function in normal turnover of these proteins and may play a role in numerous calcium-activated cellular processes.331-3323... 

Bromelain differs from (be other cysieinyl proteases papain and ficin in its 140-told difference of Itcat for the BAEK and BAA hydrolysis, suggesting a difference in the mechanism of catalysis for both substrates [37]. For BABE hydrolysis, deacylation is predominantly the rate-limiting step, while for BAA hydrolysis (he acylation is rate limiting [42]. However, Wharton et aL [43] explained the differences in kcot for BAEE and BAA hydrolysis a gauming (hat nonproductive binding plays a role in catalysis. 

Cat B is an abundant and ubiquitously expressed cysteine peptidase of the papain family and makes up a major fraction of lysosomal enzymes that is capable of degrading components of the extracellular matrix in various diseases [30-32]. Cat B is also a prognostic marker for several types of cancer [33], and increased expression and secretion of cat B has been shown to be involved in the migration and invasion of various tumours [34—36], The precise role of cat B in solid tumours is not fully understood, but it has been proposed to participate, along with other cysteine cathepsins, in metastasis, angiogenesis, and tumour progression [37], Indeed, cat B inhibitors reduce both tumour cell motility and invasiveness in vitro [38], Recently, metal complexes based on rhenium, gold and palladium were shown to be effective inhibitors of cat B [39-44],... 

The —CTanion in Ser plays the role of the catalysis so that a-chymotrypsin is a member of the class of enzymes called serine proteinase . We have many serine proteinase such as tripsin, elastase, subtilicin, etc. Papain is called thiol proteinase and contains thiol moiety in place of serine. 

Dijkman, J.P. and Duijnen P.Th. van, Papain in aqueous solution and the role of Asp-158 in the mechanism an ab initio SCF+DRF+BEM study. International Journal of Quantum Chemistry, Quantum Biology Symposium (1991) 18 49-59. 

Biological Roles of Papain-like Cysteine Proteases. 64... 

Properties of cathepsin L may be compared among papain-like cysteine cathepsins (36-39) for understanding its role in producing neuropeptides. Cathepsin L belongs to the CIA subfamily of Clan CA (36). Clan CA was formed based on recognition of the first cysteine protease papain. The crystal structure of papain shows two structural domains separated by an active-site cleft. The N-terminal domain is comprised of a-helices, and the C-terminal domain contains a -barrel. 

The strategy used by the cysteine proteases is most similar to that used by the chymotrypsin family. In these enzymes, a cysteine residue, activated by a histidine residue, plays the role of the nucleophile that attacks the peptide bond (see Figure 9.18). in a manner quite analogous to that of the serine residue in serine proteases. An ideal example of these proteins is papain, an enzyme purified from the fruit of the papaya. Mammalian proteases homologous to papain have been discovered, most notably the cathepsins, proteins having a role in the immune and other systems. The cysteine-based active site arose independently at least twice in the course of evolution the caspases, enzymes that play a major role in apoptosis (Section 2.4.3). have active sites similar to that of papain, but their overall structures are unrelated. 

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