Origin and early evolution of transition element enzymes.

In this paper we speculate on the origin and early evolution of transition element enzymes. Iron, molybdenum, and zinc, the most abundant transition elements in seawater, presumably complexed with compounds accumulated in the primeval sea in the course of chemical evolution forming compounds with subsequently evolved to form proenzymes or early enzymes with low activity and broad specificity. Iron complexes may be regarded as precursors of electron transfer enzymes, molybdenum complexes as precursors of enzymes involved in the metabolism of small molecules, and zinc complexes as precursors of hydrolytic and transferring enzymes, including enzymes participating in the metabolism of macromolecules and information transfer. The different iron, molybdenum, and zinc enzymes found in bacteria including Clostridium may then have arisen through specialization by increases in the enzyme specificity of these proenzymes. Copper would have been incorporated as an enzyme constituent after the elevation of environmental redox potential, probably due to the accumulation of atmospheric oxygen.