Plasmamembrane reductases

For other biological systems, the role of plasmamembrane reductases is more obvious. In plant eells, an electrochemical gradient across the plasmamembrane... 

In laboratory cultures of marine phytoplankton, a variety of Cu(II) complexes are reduced by plasmamembrane reductases. While we have no evidence for their activity in natural waters, the maximum contribution of these enzymes to Cu(ll) reduction can be calculated. If we assume that all Cu(II) is reducible by a plasmamembrane reductase similar to that present in T. weissflogii, and that cell densities are 10s to 107 cells per liter, then the reduction rate in natural waters would be between 2.5-250pMday-1. This reduction rate would increase twofold if the activity of the cell wall and soluble reductants were considered. In surface coastal waters, Moffett and Zika (1987) found a Cu(I) production rate of 200pMh-1. Supposing that this rate is maintained throughout the light portion of the day (10 h), then the daily production rate is 2000 pM day 10... 

The role of ectoenzymes in Fe cycling is not known presently, but we have some evidence of Fe(III) reduction in phytoplankton cultures, and Fe(III) complexes are reduced by plasmamembrane reductases. Hydrogen peroxide can oxidize Fe(II) (Waite and Morel, 1984) and probably reduce Fe(III) in light, thus implying that the biota may promote this reaction through the activity of their extracellular enzymes. 

The electrons provided in the light reaction, however, may also be directly exported from the cells and used to reduce a variety of extracellular substrates. This electron export is effected by surface enzymes (called transplasmamembrane reductases) spanning the plasmamembrane from the inside surface to the outside. They transfer electrons from an internal electron donor [chiefly NADH and NADPH see Crane et al. (1985)] to an external electron acceptor. Direct reduction of extracellular compounds by transplasmamembrane electron transport proteins is prevalent in all cells thus far examined (Fig. 2.2). Although the function of this redox system is still subject to speculation, in phytoplankton it shows considerable activity, relative to other biochemical processes. A host of membrane-impermeable substrates, including ferricyanide, cytochrome c, and copper complexes, are reduced directly at the cells surface by electrons originating from within the cell. In phytoplankton, where the source of electrons is the light reactions of photosynthesis, the other half-redox reaction is the evolution of ()2 from H20. In heterotrophs, the electrons originate in the respiration of reduced substances. 

Similar transplasmamembrane reductases are present in other algae and aquatic macrophytes, and probably aquatic bacteria. As in the preceding example, evidence for their existence comes from the observation that a variety of membrane-impermeable solutes are oxidized and reduced by intact cells. Distinct from these enzymes are other redox enzymes present on the inside and outside surfaces of the plasmamembrane that may somehow be linked to the transplasmamembrane redox system. 

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