The Bacterial View of the Periodic Table

Specific Functions for All Elements

抄録

Bacterial chromosomes have genes for transport of inorganic nutrient cations (such as NH₄⁺, K⁺, Mg²⁺, Co²⁺, Fe³⁺, Mn²⁺, Zn²⁺ and other trace cations) and oxyanions (such as PO₄^3- and SO₄^2- and less abundant oxyanions). Together these account for sometimes several hundred genes in many bacteria. Bacterial plasmids encode resistance systems for toxic metal and metalloid ions including Ag⁺, AsO₂^-, AsO₄^3-, Cd²⁺, Co²⁺, CrO₄^2-, Cu²⁺, Hg²⁺, Ni²⁺, Pb²⁺, Sb³⁺, TeO₃^2-, Tl⁺, and Zn²⁺. Most resistance systems function by energy-dependent efflux of toxic ions. A few involve enzymatic (mostly redox) transformants. Some of the efflux resistance systems are ATPases and others are chemiosmotic ion/proton exchangers. Mercury resistance is due to enzymatic detoxification with organomercurial lyase (cutting the C-Hg bond of compounds such as methylmercury and phenylmercury) and mercuric reductase (Hg²⁺→Hg⁰). The Cd²⁺-resistance cation P-type ATPases of Gram-positive bacteria drives Cd²⁺ (and Zn²⁺) efflux from resistant cells. The genes defective in the human hereditary diseases of copper metabolism, Menkes syndrome and Wilson's disease, encode Cu-specific P-type ATPases that are similar to bacterial Cd²⁺ ATPases. The arsenic resistance system transports arsenite [As(III)], alternatively with the ArsB protein functioning as a chemiosmotic efflux transporter or with two proteins, ArsB and ArsA, functioning as an ATPase transporter. The third protein of the arsenic resistance system is an enzyme that reduces intracellular arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system. In Gram negative cells, a three polypeptide complex functions as a chemiosmotic cation/proton exchanger to efflux Cd²⁺, Zn²⁺, and Co²⁺. This pump consists of an inner membrane (CzcA), an outer membrane (CzcC) and a membrane-spanning (CzcB) protein that function together.