I don't any real point to make, I just thought this was an interesting
comment from the 11Apr02 issue of Nature (p.587):
"The enzymes that catalyse such reactions in nature typically operate at
kilohertz frequencies: many biological chemical reactions
convert substrates to products at rates of roughly 1,000 per second.
Almost half of these enzymes require metal ions to achieve their
catalytic functions -- usually Mg, Ca, Mn, Fe, Co, Ni, Cu and Zn --
typically embedded in a three-dimensional protein matrix that
ensures tightly controlled alignment of the reactants to achieve the
desired formation and release of the product molecule(s). Outside
the protein milieu, these metals in their typical + 2 and + 3 oxidation
states form complexes that exchange their ligands rapidly Q
they are kinetically labile.
These ions seem, therefore, to have been adopted by living organisms
precisely because they can bind to substrates and release
products on a timescale commensurate with biological requirements. Notably
absent from the list are second- and third-row
transition metals, such as Ru, Pd and Pt. Complexes containing these metal
ions are important as industrial catalysts, but their rates
of ligand exchange are much lower than those of their first-row
counterparts. With a few exceptions, such as Mo and W, such
elements have therefore not been selected for biological functions."
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______________________________________________________________________________
Lou Hom >K'93
lhom at ocf.berkeley.eduhttp://www.ocf.berkeley.edu/~lhom/
