Oo

Il II

NH2—CH — C — NH — R' R — C — NH — CH — C — OH

Il II II

Scheme 11.5 Mechanism of degradation of aspartyl peptides in acidic media.

Racemisation

All amino acid residues except Gly are chiral at the carbon bearing the side-chain and are subject to base-catalysed racemisation. Scheme 11.4 shows the mechanism involved. In alkaline solution the hydrogen of the a-methine is removed by base to form a carban-ion intermediate which can then generate D-enantiomers, which are nonmetabolisable, or create peptide bonds which are not broken down by proteolytic enzymes.

Proteolysis

The amino acid residue which is by far the most susceptible to proteolysis is Asp; the cleavage of the peptide bonds in dilute acid proceeds at a rate at least 100 times that for other peptide bonds. The hydrolysis can occur at the N-terminal and/or the C-terminal peptide bonds adjacent to the Asp residue (Scheme 11.5). Cleavage of the N-terminal peptide bond proceeds via an intermediate with a six-membered ring, while cleavage of the C-terminal peptide bond is thought to involve a five-membered ring. Such peptide bond cleavage can result in protein inactivation.

Beta-elimination

The inactivation of proteins at high temperatures is often due to ^-elimination of disulfides from the cystine residue, although other amino acids including Cys, Ser, Thr, Phe, and Lys can be degraded via ^-elimination, as seen from Scheme 11.4. The inactivation is particularly rapid under alkaline conditions and is also influenced by the presence of metal ions.

Disulfide formation

The interchange of disulfide bonds can result in incorrect pairings with consequent changes of three-dimensional structure and loss of catalytic activity. The mechanism is thought to be different in alkaline and acid conditions. In alkaline and neutral solutions the reaction involves the nucleophilic attack on a sulfur atom of the disulfide (Scheme 11.6a). This reaction is catalysed by thiols and can be prevented if thiol scavengers such as p-

(a) Alkaline or neutral conditions :

0 0

Post a comment