Penicillopepsin catalytic mechanism

A possible catalytic mechanism of aspartic proteinases, residues numbered as for penicillopepsin... 

Penicillopepsin is an acid protease produced by the mold Penicillium janthinellum at pH s less than 4.1 (1). Enzyme production occurs after the mycelial growth has ceased and sporulation has begun (2). The specificity and catalytic mechanism of penicillopepsin are very similar to those of porcine pepsin (3). The two active site aspartic acid residues, Asp-32 and Asp-215, occur in peptide sequences of at least eight amino acid residues which are almost identical in penicillopepsin, pepsin and chymosin (1,4-10). 

Figure 8. Proposed general base catalytic mechanism of the acid proteases. "A" shows a schematic drawing of the productive binding mode for substrate RCONHR in the active site of penicillopepsin. The conformational change of the flap region brings Tyr-75 to a position from which it can protonate the amide nitrogen of the scissile bond. Asp-32 attacks the carbon atom of the polarized system through...

Figure 1. Schematic representation of the relationships between proposed catalytic and inhibitory mechanisms. A. Postulated general acid-general base catalyzed mechanism for substrate hydrolysis by an aspartyl protease. The water molecule indicated is extensively hydrogen bonded to both aspartic acid residues plus other sites in the active site (see Reference 16 for details). Hydrogen bonds to water are omitted here. B. Kinetic events associated with the inhibition of pepsin by pepstatin. The pro-S hydroxyl group of statine displaces the enzyme immobilized water molecule shown in Figure lA. Variable aspartyl sequence numbers refer to penicillopepsin (pepsin, Rhizopus pepsin), respectively.

Four reviews dealing with the mechanism of action of pepsin have been published in recent years (46, 73, 108, 117). Other recent publications deal with various aspects of this mechanism 118-120). In this section, therefore, the main emphasis will be placed on the significance of studies on hitherto unobserved pepsin- and penicillopepsin-catalyzed transpeptidation reactions, especially as they aflFect the mechanisms proposed by various authors. The question we are concerned with is the role of the two carboxyl groups which are involved in the catalytic action. We shall not further consider the role of other functional groups which have been discussed in the previous section. 

In view of the three-dimensional structure of penicillopepsin that we have observed, we are in a position to contribute to an understanding of the mechanism of acid protease catalysis. Any mechanism describing the catalytic event must, of course, be consistent with a number of known facts regarding these enzymes as deduced from chemical and enzymic studies of the cleavage of small synthetic peptide substrates. Knowles has summarized these facts (35) and has presented a possible mechanism for the action of acid proteases (30,35). More recently, Fruton (36) has reviewed the acid protease field and in the following discussion we have attempted to... 

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