Protein and peptide alkoxyl radicals can give rise to C-terminal decarboxylation and backbone cleavage

TY - JOUR

T1 - Protein and peptide alkoxyl radicals can give rise to C-terminal decarboxylation and backbone cleavage

AU - Davies, Michael Jonathan

PY - 1996/12/1

Y1 - 1996/12/1

N2 - Previous studies have demonstrated that gamma-irradiation of some free amino acids in the presence of oxygen gives high yields of side-chain hydroperoxides. It is shown in the present study that N-acetyl amino acids and peptides also give high levels of hydroperoxides on gamma-irradiation, even when the free amino acid does not, and that hydroperoxides can be formed on both the backbone (at alpha-carbon positions) and the side chain. Decomposition of alpha-carbon hydroperoxides by Fe(II)-EDTA gives initially an alkoxyl radical via a pseudo-Fenton reaction; these radicals fragment rapidly with k estimated as > or = 10(7) s(-1). With N-acetyl amino acids and dipeptides beta-scission of an alkoxyl radical at the C-terminal alpha-carbon results in C-terminal decarboxylation, with release of CO2.-; the corresponding amides undergo deamidation with release of .C(O)NH2. Cyclic dipeptides undergo analogous reactions with cleavage of the alpha-carbon to carbonyl-carbon bond and formation of .C(O)NHR radicals. With substrates with large aliphatic side chains, radicals from side-chain hydroperoxides are also observed. C-terminal decarboxylation and backbone fragmentation are also observed with larger peptides, amino acid homopolymers, and proteins. These observations suggest that alpha-carbon alkoxyl radicals may be key intermediates in the fragmentation of proteins in the presence of oxygen. The radicals released in these processes may react further to form O2.-, or redox cycle metal ions. These reactions may be propagating processes during protein chain oxidation.

AB - Previous studies have demonstrated that gamma-irradiation of some free amino acids in the presence of oxygen gives high yields of side-chain hydroperoxides. It is shown in the present study that N-acetyl amino acids and peptides also give high levels of hydroperoxides on gamma-irradiation, even when the free amino acid does not, and that hydroperoxides can be formed on both the backbone (at alpha-carbon positions) and the side chain. Decomposition of alpha-carbon hydroperoxides by Fe(II)-EDTA gives initially an alkoxyl radical via a pseudo-Fenton reaction; these radicals fragment rapidly with k estimated as > or = 10(7) s(-1). With N-acetyl amino acids and dipeptides beta-scission of an alkoxyl radical at the C-terminal alpha-carbon results in C-terminal decarboxylation, with release of CO2.-; the corresponding amides undergo deamidation with release of .C(O)NH2. Cyclic dipeptides undergo analogous reactions with cleavage of the alpha-carbon to carbonyl-carbon bond and formation of .C(O)NHR radicals. With substrates with large aliphatic side chains, radicals from side-chain hydroperoxides are also observed. C-terminal decarboxylation and backbone fragmentation are also observed with larger peptides, amino acid homopolymers, and proteins. These observations suggest that alpha-carbon alkoxyl radicals may be key intermediates in the fragmentation of proteins in the presence of oxygen. The radicals released in these processes may react further to form O2.-, or redox cycle metal ions. These reactions may be propagating processes during protein chain oxidation.

KW - Amino Acids

KW - Electron Spin Resonance Spectroscopy

KW - Free Radicals

KW - Gamma Rays

KW - Peptides

KW - Proteins

U2 - 10.1006/abbi.1996.0545

DO - 10.1006/abbi.1996.0545

M3 - Journal article

C2 - 8951048

SN - 0003-9861

VL - 336

SP - 163

EP - 172

JO - Archives of Biochemistry and Biophysics

JF - Archives of Biochemistry and Biophysics

IS - 1

ER -