Rhizopus-pepsin

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.

The sequence of penicillopepsin from Penicillium jantinellum is almost completed, while partial sequences are known for Rhizopus-pepsin (Rhizopus chinensis) and Mucor miehei proteinase. A proteinase from Endothia parasitica and the enzymes from jantinellum and R. chinensis have been solved for x-ray crystal structure (see Chapter 3, 4, and 5). 

The carboxyl proteases are so called because they have two catalytically essential aspartate residues. They were formerly called acid proteases because most of them are active at low pH. The best-known member of the family is pepsin, which has the distinction of being the first enzyme to be named (in 1825, by T. Schwann). Other members are chymosin (rennin) cathepsin D Rhizopus-pepsin (from Rhizopus chinensis) penicillinopepsin (from Penicillium janthinel-lum) the enzyme from Endothia parasitica and renin, which is involved in the regulation of blood pressure. These constitute a homologous family, and all have an Mr of about 35 000. The aspartyl proteases have been thrown into prominence by the discovery of a retroviral subfamily, including one from HIV that is the target of therapy for AIDS. These are homodimers of subunits of about 100 residues.156,157 All the aspartyl proteases contain the two essential aspartyl residues. Their reaction mechanism is the most obscure of all the proteases, and there are no simple chemical models for guidance. 

Free Energy Calculations on Enzyme-Inhibitor Complexes Studies of Thermolysin and Rhizopus Pepsin... 

Figure 2. A ribbon diagram of rhizopus pepsin (PDB code 5APR). The catalytically important Asp dyad (Asp218 and Asp35) side-chains are shown in stick diagrams. The P-hair pin flap that covers the active site cleft is located in the bottom of the diagram.

The mutation of the hydroxyl group positioned in R-configuration at the C(3) atom of the central statine (rSta) residue of the inhibitor gives rise to AAGbind of -0.51 kcal/mol, which is very close to the experimental value of -0.8 kcal/mol. It may be noted here that the starting configuration of the inhibitor in the enzyme-inhibitor complex is the same as that of pepstatin. The crystal structure of rhizopus pepsin or any other aspartic proteinase... 

B. G. Rao, and U. C. Singh, Studies on the binding of pepstatin and its derivatives to Rhizopus pepsin by quantum methanics, molecular mechanics, and free energy perturbation methods, J. Am. Chem. Soc. 113 6735 (1991). 

K. Suguna, E. A. Padlan, R. Bott, J. Boger, K. D. Parris, and D. R. Davies, Structures of complexes of rhizopus pepsin with pepstatin and other statine-containing inhibitors, Proteins 13 195 (1992). 

Finds crystal structure of peptides bound to Rhizopus pepsin... 

Rennin—see chymosin Repertoire selection 415-418 Reporter group 276 Resonance Raman spectra 476 Restriction endonucleases 406-408 Restriction fragment 408 Retention of stereochemical configuration 253, 254 Reverse genetic s 438 -442 Reverse transcriptase 3 8 Rhizopus-pepsin 486, 490 Ribonuclease A 9... 

Penicillopepsin Aspergillopeptidase A Aspergillus oryzae acid protease Rhizopus-pepsin Mucor rennins... 

It is clear that a great deal of work is still required in the field of acid proteases before we reach the level of understanding attained for other groups of proteolytic enzymes. Fortunately the amino acid sequence work on at least three enzymes—pepsin, chymosin, and penicillopepsin— is well advanced, and complete three-dimensional structures should become available in the near future. A tentative structure for rhizopus-pepsin has been obtained, but in the absence of sufficient sequence information interpretation of the electron density maps is difficult. We can... 

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