The functional role of zinc in angiotensin converting enzyme: implications for the enzyme mechanism

Abstract

Zinc is essential to the catalytic activity of angiotensin converting enzyme. The enzyme contains one g-atom of zinc per mole of protein. Chelating agents abolish activity by removing the metal ion to yield the inactive, metal-free apoenzyme. Zinc does not stabilize protein structure since the native and apoenzymes are equally susceptible to heat denaturation. Addition of either Zn 2+, Co 2+, or Mn 2+ to the apoenzyme generates an active metalloenzyme; Fe 2+, Ni 2+, Cu 2+, Cd 2+, and Hg 2+ fail to restore activity. The activities of the metalloenzymes follow the order Zn > Co > Mn. The protein binds Zn 2+ more firmly than it does Co 2+ or Mn 2+. Hydrolysis of the chromophoric substrate, furanacryloyl-Phe-Gly-Gly, by the active metalloenzymes is subject to chloride activation; the activation constant is not metal dependent. Metal replacement mainly affects k cat with very little change in K M , indicating that the role of zinc is to catalyze peptide hydrolysis.