Peptides cleaving enzymes

The identification of the morphine receptor spurred an effort in many laboratories to find an endogenous agonist for which that receptor was normally intended. Ultimately, a pair of pentapeptides that bound quite tightly to opiate receptors were isolated from mammalian brains. These peptides, called enkephalins (2, 3), show many of the activities of synthetic opiates in isolated organ systems. They do in fact show analgesic activity when injected directly into the brain. It is thought that lack of activity by other routes of administration is due to their rapid inactivation by peptide cleaving enzymes. 

Major classes of peptide-cleaving enzymes Section 9.1.6... 

Cysteine, Aspartyl, and Meta Hop ro teases Are Other Major Classes of Peptide-Cleaving Enzymes... 

The metalloproteases constitute the final major class of peptide-cleaving enzymes. The active site of such a protein contains a bound metal ion, almost always zinc, that activates a water molecule to act as a nucleophile to attack the peptide carbonyl group. The bacterial enzyme thermolysin and the digestive enzyme carboxypeptidase A are classic examples of the zinc proteases. Thermolysin, but not carboxypeptidase A, is a member of a large and diverse family of homologous zinc proteases that includes the matrix metalloproteases, enzymes that catalyze the reactions in tissue remodeling and degradation. 

Classification of Proteases. All peptide-cleaving enzymes are divided customarily into endopeptidases and exopeptidases, according to their specific mode of action. Exopeptidases are so called because they attack peptide chains only at the ends, in other words, they remove terminal amino acids only. A further distinction is made between carboxypeptidases acting on the carboxyl end and aminopeptidases acting on the amino end of the chains. The preferred substrates for exopeptidases are smaller protein fragments, oligopeptides and polypeptides hence they are also called simply peptidases. ... 

Puglielli, L., Ellis, B.C., Saunders, A.J., Kovacs, D.M. (2003) Ceramide stabdizes P-site amyloid precursor protein-cleaving enzyme-1 and promotes amyloid P-peptide biogenesis. /. Biol. Chem., 278, 19777-19783. 

Beta amyloid peptide is cleaved from the beta amyloid precursor protein by the beta secretase (or beta amyloid cleaving enzyme, BACE) and gamma secretase enzymes (Cummings, 2004). Experimental studies in animal models... 

Hansen, L.A., Mashah, E., Galasko, D. and Terry, R.D. 1993. Plaque-only Alzheimer disease is usually the Lewy body variant, and vice versa. J. Neuropathol. Exp. Neurol. 52 648-654 Harada, H., Tamaoka, A., Ishii, K., Shoji, S., Kametaka, S., Kametani, F., Saito, Y. and Murayama, S. 2006. P-Site APP cleaving enzyme 1 (BACEl) is increased in remaining neurons in Alzheimer s disease brains. Neurosci. Res. 54 24—29 Hardy, J.A. and Selkoe, D.J. 2002. The amyloid hypothesis of Alzheimer s disease Progress and problems on the road to therapeutics. Science 297 353-356 Harris, M.E., Wang, Y, Pedigo, N.W., Jr., Hensley, K., Butterfield, D.A. and Carney, J.M. 1996. Amyloid P peptide (25-35) inhibits Na-dependent glutamate uptake in rat hippocampal astrocyte cultures. J. Neurochem. 67 277-286... 

In New York, Bergmann continued his investigations of amino acids, peptides and proteins on a wider base. This was done through intensive work on analytical methods for the determination of amino acids and peptides and also through an enhanced examination of protein-cleaving enzymes with the necessary synthesis of peptides suitable for serving as model substrates. 

The enzyme works optimally at pH > 9 with N-protected amino acid esters as acyl donors, and amino acids or amino acid amides as nucleophiles (acceptors). At that pH value it has high esterase activity an amino acyl-enzyme complex will be formed rapidly which transfers the acyl residue to the acceptor molecule (transpeptidation), and since at alkaline pH the rate of peptide cleaving is minimal, the product formed will be released in excellent yield. As an example the last coupling step in a synthesis of the opioide peptide Met-enkephalin that started with benzoylarginine ethylester may be shown (Fig. 8). 

There are two tactics that have been widely used to inhibit viral proteolysis these are alteration of primary structure and/or conformation of the precursors, and the inactivation of the cleaving enzymes. Some of the inhibitory treatments are particularly useful in demonstrating that proteolysis is actually occurring, and in accumulating precursor proteins for comparative peptide analyses with stable end products. 

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