Keto adds

Alpha amino acids (and aldehydes) synthesis by reaction of an alpha keto add with another amino acid (Herbst-EngeO or by reaction of a keto acid with ammonia tmdef reducing conditions (Knoop-Oe sterling). 

Lipoic acid is an acyl group carrier. It is found in pyruvate dehydrogenase zard a-ketoglutarate dehydrogenase, two multienzyme complexes involved in carbohydrate metabolism (Figure 18.34). Lipoie acid functions to couple acyl-group transfer and electron transfer during oxidation and decarboxylation of a-keto adds. 

Deprotonation of the acidic a carbon of the amino acid gives an intermediate a-keto add imine. .. 

Keto add-dependent enzymes are involved in numerous reactions and are found in prokaryotes as well as eukaryotes. Apart from scopolamine and clavulanic add biosynthesis, a-keto add-dependent enzymes are also found in the biosynthetic... 

Two reactions for the production of L-phenylalanine that can be performed particularly well in an enzyme membrane reactor (EMR) are shown in reaction 5 and 6. The recently discovered enzyme phenylalanine dehydrogenase plays an important role. As can be seen, the reactions are coenzyme dependent and the production of L-phenylalanine is by reductive animation of phenylpyruvic add. Electrons can be transported from formic add to phenylpyruvic add so that two substrates have to be used formic add and an a-keto add phenylpyruvic add (reaction 5). Also electrons can be transported from an a-hydroxy add to form phenylpyruvic add which can be aminated so that only one substrate has to be used a-hydroxy acid phenyllactic acid (reaction 6). 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

KetO Adds, Aldehydes, and Esters see also Dicarbonyl Compounds)... 

In keto adds, carboxyl was reduced preferentially to the carbonyl with borane in tetrahydrofuran. 4-Phenyl-4-ketobutanoic acid afforded 4-phenyl-4-ketobutanol in 60% yield [971]. 

Keto adds were reduced more frequently to hydroxy acids or to acids. On... 

Karl-Fischer titration, 80, 82 o-Keto add 2,4-dinitr<9benylhydrazonet, 265... 

Phytochemical Reduction of Keto Adds and Keto Aldehydes... 

The a-keto adds and their metabolic byproducts are excreted in urine, and these compounds cause the characteristic sweet, "maple syrup"aroma. 

Interest in the total synthesis of the Aspergillus terreus derived quadrone fi06), an antitumor agent has been very intense. Success was first realized in Danishefsky s laboratory Once 601 was reached, its sidechain was elaborated and ring closure effected (Scheme LII). Condensation of 602 with 1-tert-butoxy-l-tcrt-butyl-dimethylsiloxyethylene in the presence of titanium tetrachloride and subsequent desilylation resulted in introduction of an angular acetic acid moiety. The two sidechains were next connected by intramolecular alkylation and the resulting keto add was subjected to selenenylation in order to produce 603. The a, P-unsaturated double bond was used to force enolization to the a position. Indeed, 604 was... 

V(C104)2 or-Keto adds Tartaric acid derivatives 10... 

OXIDATIVE DECARBOXYLATION (Branched-chain a-keto add dehydrogenase Coenzymes TPP, CoA, lipdcacid, NAD, FAD)... 

Thiamine pyrophosphate (TPP) is the biologically active form of fre vitamin, formed by the transfer of a pyrophosphate group from ATP to thiamine (Figure 28.11). Thiamine pyrophosphate serves as a coen zyme in the formation or degradation of a-ketols by transketolase (Figure 28.12A), and in the oxidative decarboxylation of a-keto adds i (Figure 28.12B). 

Tetrahydro-l,2-diazocinones (166) are formed in good yield by cyclization of e-keto adds with hydrazine in dilute solution. Standard transformations lead to the hexahydro derivatives (167) and (168) and the octahydrodiazocine (169) (73BSF2029). 

Heat-generated flavors, due to the formation of lactones and methyl ketones from hydroxy and keto add precursors, which occur in trace quantities in milk lai. These flavors arc considered to be desirable in fried and baked goods and are partly responsible for the unique condiment properties of butler in food preparation. However, they are undesirable in dried whole milk and evaporated milk where the objective is to make a bland product as much like fresh milk as possible. 

Butanone, conversion to a-ethyl-a-methylsuccinic acid, 44, 59 Butenolides from aryl aldehydes and y-keto adds, 43, 5 7-(-Butoxynorbornadifne, 44,12... 

A combination of D-amino acid oxidase and L-amino transferase is an example of a deracemization by stereoinversion. The product is an L-amino acid. The reaction catalyzed by amino transferase has an equilibrium constant close to unity, a very unpractical situation leading to uncomplete transformation and to the production of almost inseparable mixtures of amino acids (at least two, the amino acid product and the amino add used as an amino donor). For preparative purposes it is therefore mandatory to shift the equihbrium to the product side. A recent example of a deracemization procedure based on this coupled enzymatic system is the preparation of L-2-naphthyl-alanine 6 as illustrated in Scheme 13.9 [28]. The reaction occurs in one pot with initial oxidation of the D-amino acid catalyzed by D-amino acid oxidase from Rhodotonda gracilis. The hydrogen peroxide that is formed in stoichiometric amounts is decomposed by catalase. The a-keto add is the substrate for L-aspartate amino transferase (L-Asp amino transferase), which is able to use L-cysteine sulfinic acid 7 as an amino donor. 

Recently, L-amino acid deaminase (EC 1.4.3.x) activities have been identified, particularly from the Proteus genus [59]. This enzyme, constituted by 370 residues, is an FAD-containing L-amino acid oxidase flavoprotein that uses molecular oxygen to convert L-amino acids into the corresponding a-keto adds and ammonia but does not produce hydrogen peroxide. L-amino acid deaminase prefers amino acids with aliphatic, aromatic, and sulfur-containing side chains (the best substrates are L-heu, L-Phe, L-Met, and L-Trp) and its kinetic efficiency is quite low because of the low Vnm value (<2 units/mg protein). 

Most amino nods lose their nitrof en atom by a IransamiiiatiDa rent tion in which the -NH group of the amino acid changes places with the keto group of n-ketoglutarate. The products are a new n-keto add and glutamate ... 

Problwi 29 11 What -keto add u fornoed on uanaamination of leuciae ... 

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