Energy-linked transhydrogenase

Energy-linked transhydrogenase, a protein in the inner mitochondrial membrane, couples the passage of protons down the electrochemical gradient from outside to inside the mitochondrion with the transfer of H from intramitochondrial NADH to NADPH for intramitochondrial enzymes such as glutamate dehydrogenase and hydroxylases involved in steroid synthesis. 

Steroid-metabolizing mitochondria, e.g., those from beef adrenal cortex (76, 82-84) and porcine corpus luteum (86, 86), were recently shown to contain an active energy-linked transhydrogenase that appears to play a role in steroid hydroxylation reactions (see Section IV). Under proper assay conditions all mitochondrial transhydrogenases hitherto described may be coupled to respiratory energy or ATP. 

Among respiring bacteria evidence for an energy-linked transhydrogenase was provided by Murthy and Brodie (60) and by Fisher and Sanadi (61) with membrane fragments from Escherichia coli. A similar but apparently non-energy-linked transhydrogenase was also found in Myco-... 

Fio. 1. Relationship of the energy-linked transhydrogenase reaction to oxidative phosphorylation. From Ernsteret al. 166). 

Since the equilibrium constant of the non-energy-linked transhydrogenase reaction is 0.79 (SO) and that of ATP hydrolysis is about 10 M... 

At neutral pH, the maximal initial velocities of the two directions of the non-energy-linked transhydrogenase reaction differ by a factor of about five, the reduction of NADP being the slower reaction (SO, 69, 68, 71, 127). Reduction of NADP by NADH is maximally active at about pH 5.5., whereas reduction of NAD by NADPH shows a pH optimum at about 7.0 (SO, 67, 71, 72 see also S2). When the reduction of NADP by NADH approaches equilibrium, the rate constant of the reaction is increased (67), indicating an activation of the transhydrogenase that is related to the accumulation of the products NAD and NADPH. It has been proposed (67, 69, 71) that this activation may involve a conversion of the enzyme from an inactive to an active conformational state, similar to that proposed to occur upon energization (see below). 

As pointed out by Krebs and Veech (176), the relationship between the redox states of NAD and NADP in mammalian cells would be governed to a large extent by the substrate levels of NAD- and NADP-dependent dehydrogenases, interlinked by shared reactants. The coordination of these systems of interlinked dehydrogenases and, in particular, energy-linked transhydrogenase has been a matter of controversy. Funda-... 

Energy-linked affinity changes also seem to be of importance for the effect of certain inhibitors of transhydrogenase, e.g., metal ions [45,80]. The energy-linked transhydrogenase reaction catalyzed by submitochondrial particles is known to be inhibited by Mg " to a lesser extent than the nonenergy-linked reaction [45]. In addition, the effect of is pH-dependent with an increasing effect of the... 

Early proposals concerning the mechanism of the energy-linked transhydrogenase reaction were based on the chemical hypothesis of oxidative phosphorylation [82] and visualized the involvement of high-energy intermediates of the type 1 X, NADH I, NADP I, etc. [29,46]. These proposals, however, just as the chemical hypothesis as a whole, had to be abandoned because of lack of experimental evidence. 

Fig. 7.2. Alternative-site model of energy-linked transhydrogenase. E denotes energized form of transhydrogenase. > E and E <, and < E and E > denote tight and loose binding, respectively, of substrate/products to energized transhydrogenase.

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