Amino-FAD

FIGURE 11.4 Electrical wiring of redox enzymes, (a) Optimal configuration for the electrical contacting of a redox enzyme with the electrode, (b) Reconstitution of an apoenzyme on a relay-cofactor monolayer for the alignment and electrical wiring of a redox enzyme. The structures of the redox relay molecule PQQ and cofactor amino-FAD are shown in the inset. (Reproduced with permission from Ref. [53]. Copyright 2006, Elsevier.)... 

Riboflavin-5 -Adenosine Diphosphate. Riboflavin-5 -adenosine diphosphate [146-14-5] (flavin—adenine dinucleotide, FAD), C27H33N9O15P2 (2), mol wt 785.56, was first isolated in 1938 from the D-amino acid oxidase as its prosthetic group (95), where it was postulated to be... 

Riboflavin was first isolated from whey in 1879 by Blyth, and the structure was determined by Kuhn and coworkers in 1933. For the structure determination, this group isolated 30 mg of pure riboflavin from the whites of about 10,000 eggs. The discovery of the actions of riboflavin in biological systems arose from the work of Otto Warburg in Germany and Hugo Theorell in Sweden, both of whom identified yellow substances bound to a yeast enzyme involved in the oxidation of pyridine nucleotides. Theorell showed that riboflavin 5 -phosphate was the source of the yellow color in this old yellow enzyme. By 1938, Warburg had identified FAD, the second common form of riboflavin, as the coenzyme in D-amino acid oxidase, another yellow protein. Riboflavin deficiencies are not at all common. Humans require only about 2 mg per day, and the vitamin is prevalent in many foods. This vitamin... 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

Protein-protein interactions between heterodimeric protein pairs that form only transient interactions can be detected, y-secre-tase is presenilin-1 (PS1) dependent [51-53]. PS1 is a 467-amino acid, 9-transmembrane domain protein. Over 100 documented single point mutations are known to cause autosomal-dominant familial AD (FAD) [54], in which the ratio of the more fibrilogenic variety of A ft (A/142) to the less fibrilogenic variety (A/140) is increased. Chinese hamster ovary (CHO) cells were stably transfected with human APP and either wild type or mutant PS1 [4, 55]. 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

Flavin Coenzymes.—5-Deazaflavin-adenine dinucleotide (2) can be prepared from the 5-deazaFMN,21 using a FAD pyrophosphorylase from rat liver.22 When the apoprotein of D-amino-acid oxidase from pig kidney is reconstituted with (2), no oxidation of D-alanine is observed, although the flavin chromophore in the reconstituted enzyme is reduced on the addition of DL-amino-acids.22 This has been interpreted as indicating that hydrogen transfer from the amino-acid to (2) can still... 

Many of the amino acids originally tested by Krebs were racemic mixtures. When naturally occurring L-amino acids became available the oxidase was found to be sterically restricted to the unnatural, D series. [D-serine occurs in worms free and as D-phosphoryl lombricine (Ennor, 1959)]. It could not therefore be the enzyme used in the liver to release NH3 in amino acid metabolism. D-amino acid oxidase was shown by Warburg and Christian (1938) to be a flavoprotein with FAD as its prosthetic group. A few years later Green found an L-amino acid oxidase in liver. It was however limited in its specificity for amino acid substrates and not very active—characteristics which again precluded its central role in deamination. 

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