Liver coenzyme binding

A crystal structure of a ternary complex of horse liver alcohol dehydrogenase with NADH and the inhibitor, dimethyl sulfoxide, first at 4.5 A resolution1365 and a further refinement to 2.9 A resolution,1366 has been published by Eklund et al. The gross structure of the ternary complex is similar to that of the free enzyme structure. Each subunit is divided into a coenzyme-binding domain and a catalytic domain. The subunits are joined together near the... 

The steady state and stopped-flow kinetic studies on the horse liver enzyme are now considered classic experiments. They have shown that the oxidation of alcohols is an ordered mechanism, with the coenzyme binding first and the dissociation of the enzyme-NADH complex being rate-determining.15,26,27 Both the transient state and steady state methods have detected that the initially formed enzyme-NAD+ complex isomerizes to a second complex 27,28 In the reverse reaction, the reduction of aromatic aldehydes involves rate-determining dissociation of the enzyme-alcohol complex,27,29 whereas the reduction of acetaldehyde is... 

Liver alcohol dehydrogenase subunit viewed as a CPK model. The left-hand side of the molecule is the coenzyme binding domain and the right-hand side is the catalytic domain. The catalytic zinc ion is accessible from two channels located above (not visible) and below the coenzyme binding domain. The upper channel permits approach of the nicotinamide ring of the coenzyme. The lower channel permits approach of the substrate. The substrate channel closes up, trapping the substrate inside the molecule, when both coenzyme and substrate are present. 

Preliminary efforts to resolve TK from rat liver or human red blood cells into apo-enzyme and TPP were unsuccessful indicating a very tight coenzyme binding [7]. Therefore, the enzyme from Baker s yeast was chosen, because it was already known from Racker s work that holo-TK can easily be separated into apo-enzyme and TPP. The availability of pure apo-TK was considered as a prerequisite for the studies on the coenzyme binding. 

Kvassman, J. Pettersson, G. (1979). Effect of pH on coenzyme binding to liver alcohol dehydrogenase. European Journal of Biochemistry, 100, 115-23. 

Heme, an iron-containing tetrapyrrole pigment, is a component of 02-binding proteins (see p. 106) and a coenzyme of various oxi-doreductases (see p. 32). Around 85% of heme biosynthesis occurs in the bone marrow, and a much smaller percentage is formed in the liver. Both mitochondria and cytoplasm are involved in heme synthesis. 

NAD glycohydrolases from rat liver nuclei, 66, 151 poly(ADP-ribose) synthetase from rat liver nuclei, 66, 154 poly(ADP-ribose) synthetase from calf thymus, 66, 159 extraction and quantitative determination of larger than tetrameric endogenous polyadenosine diphosphoribose from animal tissues, 66, 165 covalent modification of proteins by metabolites of NAD, 66, 168 coenzyme activity of NAD bound to polymer supports through the adenine moiety, 66, 176 use of differently immobilized nucleotides for binding NAD -dependent dehydrogenases, 66, 192. 

Mechanism of Action Acts as a coenzyme for various metabolic functions, including fat and carbohydrate metabolism and protein synthesis. Therapeutic Effect Necessary for cell growth and replication, hematopoiesis, and myelin synthesis. Pharmacokinetics In the presence of calcium, absorbed systemically in lower half of ileum. Initially, bound to intrinsicfactor this complex passes down intestine, binding to receptor sites on ileal mucosa. Protein binding High, Metabolized in the liver. Primarily eliminated unchanged in urine. Half-life 6 days. 

Each of the forms of ETF isolated from the different sources contain FAD as coenzyme and form an anionic semiquinone on one-electron reduction. Stopped-flow kinetic studies on the pig liver ETF showed the anionic flavin semiquinone to be formed at times faster than catalytic turnover and thus demonstrate the participation of the anionic FAD semiquinone as an intermediate in the acceptance of reducing equivalents from the dehydrogenase. These studies would also imply the intermediacy of the semiquinone form of the acyl CoA dehydrogenase which would have been expected to form a neutral flavin semiquinone at the time the studies of Hall and Lambeth were performed, however, no spectral evidence for its formation were found. Recent studies have shown that the binding of CoA analogs to the dehydrogenase results in the perturbation of the pKa of the FAD semiquinone such that an anionic (red) rather than the neutral (blue) semiquinone is formed. This perturbation was estimated to reduce the pKa by at least 2.5 units to a value of... 

A third group of lipid-binding proteins have a four-helix bundle structure. They include the insect lipophorins, which transport diacylglycerols in the hemolymph (see main text), and nonspecific lipid carriers of green plants.q An 87-residue four-helix protein with a more open structure binds acyl-coenzyme A molecules in liver.r... 

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