Pyridine nucleotide coenzymes, complexes

Frey PA (1987) Complex pyridine-dependent transformations. In Dolphin D, Poulson R, Avamovic O (eds) Pyridine nucleotide coenzymes Chemical biochemical, medical aspects Vol 2B. Wiley, New York, 462... 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

Mitochondria contain ubiquinone (also known as coenzyme Q), which differs from plastoquinone A (Chapter 5, Section 5.5B) by two methoxy groups in place of the methyl groups on the ring, and 10 instead of 9 isoprene units in the side chain. A c-type cytochrome, referred to as Cyt Ci in animal mitochondria, intervenes just before Cyt c a h-type cytochrome occurring in plant mitochondria is involved with an electron transfer that bypasses cytochrome oxidase on the way to 02. The cytochrome oxidase complex contains two Cyt a plus two Cyt a3 molecules and copper on an equimolar basis with the hemes (see Fig. 5-16). Both the Fe of the heme of Cyt a3 and the Cu are involved with the reduction of O2 to H20. Cytochromes a, >, and c are in approximately equal amounts in mitochondria (the ratios vary somewhat with plant species) flavoproteins are about 4 times, ubiquinones 7 to 10 times, and pyridine nucleotides 10 to 30 times more abundant than are individual cytochromes. Likewise, in chloro-plasts the quinones and the pyridine nucleotides are much more abundant than are the cytochromes (see Table 5-3). 

Wallenfels K and Sund H, Mechanism of hydrogen transfer with pyridine nucleotides. III. Binary and ternary zinc complexes as models for enzyme-coenzyme bonds, Biochem. Z., 329, 41-47 (1957). 

NAD is the coenzyme of alcohol dehydrogenase. It serves in the reversible enzymic reaction as an acceptor of the hydrogen from the substrate and is converted to NADH. This occurs in a ternary complex of enzyme, pyridine nucleotide, and substrate, in which coenzyme and substrate interact closely with amino acid residues of the enzyme, and thereby are activated sufficiently to allow reaction. 

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