Co-carboxylase

The way in which thiamine participated in the oxidation of pyruvate became clearer when Lohmann and Schuster (1937) showed vitamin Bj to be present intracellularly as thiamine pyrophosphate. In yeast, decarboxylation of pyruvate yielded ethanal which was reduced by alcohol dehydrogenase to give ethanol. A cofactor was needed for this decarboxylation, co-carboxylase. Like the cofactor needed in animal cells for the decarboxylation of pyruvate, cocarboxylase was found to be identical to thiamine pyrophosphate. Vitamin Bj thus became the first vitamin whose intracellular function as a coenzyme had been established in vitro. Another aphorism therefore arose about vitamins—B vitamins are (parts of) coenzymes—an idea that was to be completely confirmed. 

Clostridium welchii, enzymes, effect on blood group substances, IV, 55 Coacervation, I, 277 Coatings, cellulose ester, I, 323 Co-carboxylase, II, 124 Coccidioides immitis, polysaccharide formed by, II, 225 Cocositol, III, 60 Coir, xylan peroentage in, V, 271 Collagenaae, effect on blood group substances, IV, 55... 

Insulin, a hormone associated with the parasympathic nervous system, has the opposite effect. It decreases the intracellular level of c-AMP. Under these conditions PKA is deactivated and HSL is dephosphorylated by the corresponding phosphatase. Lipolysis in adipocytes is blocked and the amount of NEFAs available for -oxidation is reduced. At the same time lipogenesis is activated by the stimulation of cAMP-independent kinases, which phosphorylate and activate acetyl-Co carboxylase. 

VITAMIN Bj (Thiamine, aneurine) Vitamin Bj has an important function in carbohydrate-metabolism as a prosthetic group in the enzyme co-carboxylase. 

Auhagen, E., 1932. Co-carboxylase, ein neues Co-enzyme der alkoholischen Garung. Hoppe-Seyler s Zeitschrift fur physiologische Chemie. 204 149-167. Barker, H.A., Weissbach, H., and Smyth, R.D., 1958. A coenzyme containing pseudovitamin B12. Proceedings of the National Academy of Sciences of the United States of America. 44 1093-1097. 

Thiamine, biotin, pantothenic acid, riboflavin and vitamin B12 are involved in propionic acid fermentation. Biotin forms the prosthetic group of methyl-malonyl-CoA transcarboxylase pantothenate is a constituent of CoA thiamine is not the coenzyme (co-carboxylase) of the enzyme carboxylase like in other organisms, for acetaldehyde has not been detected in propionibacteria (although traces were recently found), but it may function as a component of dehydrogenases in oxidative phosphorylation of a-keto acids. Riboflavin is a constituent of FAD and FMN. Propionibacteria can synthesize vitamins B2 and B in considerable amounts (see below), but the other three vitamins must be supplied. Some strains can grow in synthetic media without thiamine (Silverman and Workman, 1939 Delwiche, 1949), in some other strains thiamine can be replaced by / -aminobenzoic acid. 

Phosphoflamns.—Co-carboxylase, a phosphoric ester of riboflavin. Cytoflave, the phosphoric ester of riboflavin, forming the prosthetic group in flavoprotein. 

Carboxylases are responsible for the ultimate liberation of CO2 in all biological oxidations. Their activity requires the presence of the co-enzyme, co-carboxylase or vitamin B pyrophosphate. 

Tbiamine, aneurin or vitamin Bj is directly concerned with intermediate carbohydrate metabolism, and the extensive work of R. Peters has shown that in the form of its diphosphoric ester (co-carboxylase), it works in conjunction with the dicarboxylic acid system of Szent-Gy5rgyi in brii ing about the characteristic oxidation of pyruvate in brain tissue. 

Co-carboxylase, the pyrophosphoric ester of vitamin B, is a characteristic constituent of yeast, where it co-acts with an enzyme in the decarboxylation of p3Tuvic acid to acetaldehyde and carbon dioxide during fermentation (p. 298). Iii the organism, vitamin Bj circulates in free form or as the monophosphate, and is converted to the co-enzyme locally within the tissues. The co-carboxylase in the blood is restricted entirely to the blood cells. 

Pyrimidine Vitamins.— The pyrimidine ring occurs in vitamin Bj, thiamine, and in vitamin Bj, riboflavin (pp. 206, 255), and in the co-enzyme, co-carboxylase, which is a phosphoric ester of thiamine (p. 252). Oxidation of thiamine leads to the formation of Oiiochrome, a pigment found in yeast, and characterised by its fluorescence in ultra-violet light. 

Niven and Smiley (272a) report that the intact thiamin molecule was required by 20 different strains of Streptococcus saiivarius co-carboxylase (thiamin diphosphate) was 40% more active than thiamin, on a molar basis. 

Buchman, Heegard, and Bonner (48) observed a very interesting inhibition of the activity of a carboxylase system by the pyrophosphate of the vitamin thiazole (4-methyl-5-/3-hydroxyethyl-thiazole pyrophosphate). This inhibition was explained as due to a competition between cocarboxylase (thiamin pyrophosphate) and the thiazole pyrophosphate, for the carboxylase protein, the thiazole pyrophosphate giving an inactive enzyme analogue. On the other hand, Niven and Smiley (272a) found that co-carboxylase was 40% more active than intact thiamin for Streptococcus salivarius. (See also Sarett and Cheldelin, addl. ref., 325a). 

Glncose-6-phosphatase Biotin (biocytin) CO, Propionyl-CoA carboxylase... 

Substrate availability for certain reactions can be optimized by anaplerotic ( topping-up ) reactions. For example, citrate synthase is a key control point of the TCA cycle. The co-substrates of citrate synthase are acetyl-CoA and oxaloacetate (OAA) and clearly, restriction in the availability of either substrate will decrease the rate of the citrate synthase reaction. Suppose, for example, a situation arises when acetyl-CoA concentration is significantly higher than that of OAA, the concentration of the latter can be topped-up and the concentration of acetyl-CoA simultaneously reduced by diverting some of the pyruvate away from acetyl-CoA synthesis (via pyruvate dehydrogenase) to OAA synthesis (via pyruvate carboxylase) as shown in Figure 3.1. The net effect is to balance the relative concentrations of the two co-substrates and thus to promote citrate synthase activity. 

Carbonic anhydrase (CA, also called carbonate dehydratase) is an enzyme found in most human tissues. As well as its renal role in regulating pH homeostasis (described below) CA is required in other tissues to generate bicarbonate needed as a co-substrate for carboxylase enzymes, for example pyruvate carboxylase and acetyl-CoA carboxylase, and some synthase enzymes such as carbamoyl phosphate synthases I and II. At least 12 isoenzymes of CA (CA I—XII) have been identified with molecular masses varying between 29 000 and 58 000 some isoenzymes are found free in the cytosol, others are membrane-bound and two are mitochondrial. 

To investigate the cofactor requirement and the characteristics of the enzyme, the effects of additives were examined using phenylmalonic acid as the representative substrate. The addition of ATP or ADP to the enzyme reaction mixtures, with or without coenzyme A, did not enhance the rate of reaction. From these results, it is concluded that these co-factors are not necessary for this decarboxylase. It is well estabhshed that avidin is a potent inhibitor of the bio-tin-enzyme complex [11 -14]. In the present case, addition of avidin has no influence on the decarboxylase activity, indicating that the AMDase is not a biotin enzyme. Thus, the co-factor requirements of AMDase are entirely different from those of known analogous enzymes, such as acyl-CoA carboxylases [15], methyhnalonyl-CoA decarboxylases [11] and transcarboxylases [15,16]. 

Coumaroyl-CoA is produced from the amino acid phenylalanine by what has been termed the general phenylpropanoid pathway, through three enzymatic conversions catalyzed by phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate CoA ligase (4CL). Malonyl-CoA is formed from acetyl-CoA by acetyl-CoA carboxylase (ACC) (Figure 3.2). Acetyl-CoA may be produced in mitochondria, plastids, peroxisomes, and the cytosol by a variety of routes. It is the cytosolic acetyl-CoA that is used for flavonoid biosynthesis, and it is produced by the multiple subunit enzyme ATP-citrate lyase that converts citrate, ATP, and Co-A to acetyl-CoA, oxaloacetate, ADP, and inorganic phosphate. ... 

Biotin (6.24) consists of an imidazole ring fused to a tetrahydrothiophene ring with a valeric acid side chain. Biotin acts as a co-enzyme for carboxylases involved in the synthesis and catabolism of fatty acids and for branched-chain amino acids and gluconeogenesis. 

During a fast, the liver is flooded with fatty acids mobilized from adipose tissue. The resulting elevated hepatic acetyl CoA produced primarily by fatty acid degradation inhibits pyruvate dehydrogenase (see p. 108), and activates pyruvate carboxylase (see p. 117). The oxaloacetate thus produced is used by the liver for gluconeogenesis rather than for the TCA cycle. Therefore, acetyl Co A is channeled into ketone body synthesis. 

Correct answer = D. Malonyl CoA (three carbons) is synthesized from acetyl CoA (two carbons) by the addition of CO2, using the enzyme acetyl CoA carboxylase. Because CO. is subsequently removed during fatty acid synthesis, the radioactive label will not appear at any position in newly synthesized fatty acids. 

Figure 1.35 Schematic diagram of the phenolic biosynthetic pathway accompanied by the key enzymes involved. Enzyme abbreviations PAL, phenylalanine ammonia-lyase BA2H, benzoic acid 2-hydroxylase C4H, cinnamate 4-hydroxylase COMT-1, caffeic/5-hydroxyferulic acid O-methy I transferase 4CL, p-co um a ra te C o A ligase F5H, ferulate 5-hydroxylase GT, galloyltransferase ACoAC, acetylCoA carboxylase.

Biotin (60), a water-soluble vitamin with widespread application in the growing market for health and nutrition, acts as a co-factor for carboxylase enzymes and its essential fatty acid synthesis. The key step in the chemical synthesis of biotin is the asymmetric reduction of the tetrasubstituted olefins 61 by in situ Rh(I)-4i catalyst (Scheme 12.1 S).79-83-85-86 Substrate-to-catalyst ratios of 2000 with diastereoselectivities of 99% de were achieved with Rh-4i at the multi-ton scale before production was terminated.87... 

The synthesis of several lipogenic enzymes is stimulated in the liver by thyroid hormones. For instance the concentration of acetyl Co A carboxylase, fatty acid synthetase and malic enzyme is increased in vivo after T3 injection [66-69],... 

I have been pursuing enzyme mimics, artificial enzymes that perform biomimetic chemistry, since starting my independent career in 1956. In the first work [52-59] my co-workers and I studied models for the function of thiamine pyrophosphate 1 as a coenzyme in enzymes such as carboxylase. We discovered the mechanism by which it acts, by forming an anion 2 that we also described as a stabilized carbene, one of its resonance forms. We examined the related anions from imidazolium cations and oxazolium cations, which produce anions 3 and 4 that can also be described as nucleophilic carbenes. We were able to explain the structure-activity relationships in this series, and the reasons why the thiazolium ring is best suited to act as a biological... 

Most plants reduce CO2 to carbohydrate according to the well-known Calvin-Benson or C3 pathway, where the initial product of photosynthesis is the 3C compound phosphoglycerate. Fixation of CO 2 to phosphoglycerate occurs with the assistance of the enzyme ribulose bisphosphate (RuBP) carboxylase, which discriminates heavily against C02 (11). Consequently, plants with C3 photosynthesis have 6 values that average -27.0 (12). Plants with the Hatch-Slack or Ci,... 

CO, Fixation to ribulose-1,5-bisphosphate (C-site car-boxylation) Ribulose-1,5-bisphosphate carboxylase/ oxygenase (mbisco) 3-Phosphoglycerate (2 times) Green plants, chemolrthoautotro- phs Mg Ni (see above)... 

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