Chemical modification of hemoglobin improves biocatalytic oxidation of PAHs

Chemical modifications on human hemoglobin were performed with the aim to c hange both surface and active-site hydrophobicities. The modifications incl uded covalent coupling of poly(ethylene)glycol (5000 MW) on free amino grou ps and the methyl esterification of free carboxylic groups. The modified he moglobin was assayed for the oxidation of 11 polycyclic aromatic hydrocarbo ns (PAHs) and 2 organosulfur aromatic compounds, Acenaphthene, anthracene, azulene, benzo(a)pyrene, fluoranthene, fluorene, phenanthrene, and pyrene w ere transformed to their respective quinones, while for chrysene and biphen yl no biocatalytic reaction could be detected. Dibenzothiophene and thianth rene were oxidized to form sulfoxides. The doubly modified hemoglobin, PEG- Methemoglobin, showed up to 10 times higher activity than the unmodified pr otein, The kinetic constants show that the PEG-Met-hemoglobin has a signifi cantly higher catalytic efficiency. The equilibrium substrate binding const ants for unmodified and PEG-Met-modified hemoglobis and hemoglobin show tha t this catalytic enhancement could be attributed to the affinity increase f or hydrophobic substrates in the modified protein. (C) 2000 Academic Press.