Abstract
This review focuses on the effects of hydrogen sulfide (H(2)S) on the unique bioenergetic molecular machines in mitochondria and bacteria-the protein complexes of electron transport chains and associated enzymes. H(2)S, along with nitric oxide and carbon monoxide, belongs to the class of endogenous gaseous signaling molecules. This compound plays critical roles in physiology and pathophysiology. Enzymes implicated in H(2)S metabolism and physiological actions are promising targets for novel pharmaceutical agents. The biological effects of H(2)S are biphasic, changing from cytoprotection to cytotoxicity through increasing the compound concentration. In mammals, H(2)S enhances the activity of F(o)F(1)-ATP (adenosine triphosphate) synthase and lactate dehydrogenase via their S-sulfhydration, thereby stimulating mitochondrial electron transport. H(2)S serves as an electron donor for the mitochondrial respiratory chain via sulfide quinone oxidoreductase and cytochrome c oxidase at low H(2)S levels. The latter enzyme is inhibited by high H(2)S concentrations, resulting in the reversible inhibition of electron transport and ATP production in mitochondria. In the branched respiratory chain of Escherichia coli, H(2)S inhibits the bo(3) terminal oxidase but does not affect the alternative bd-type oxidases. Thus, in E. coli and presumably other bacteria, cytochrome bd permits respiration and cell growth in H(2)S-rich environments. A complete picture of the impact of H(2)S on bioenergetics is lacking, but this field is fast-moving, and active ongoing research on this topic will likely shed light on additional, yet unknown biological effects.