3-hydroxybutyrate biosynthesis

Theodorou, M. C. Panagiotidis, C. A. Panagiotidis, C. H. Pantazaki, A. A. and Kyriakidis, D. A. Involvement of the AtoS-AtoC signal transduction system in po-ly-(R)-3-hydroxybutyrate biosynthesis in Escherichia coli. Biochim. Biophys. Acta. 2006,1760(6), 896-906. 

Senior P J, Dawes E A. (1971), Poly(/3-hydroxybutyrate) biosynthesis and the regulation of glucose metabolism in Azotobacter beijerinckii , Biochem J, 125, 55-66. 

O.P. Peoples, A.J. Sinskey, Poly-beta-hydroxybutyrate biosynthesis in Alcaligenes eutrophus H16. Characterization of the genes encoding beta-ketothiolase and acetoacetyl-CoA reductase, J. Biol. Chem. 264 (26) (1989) 15293-15297. 

Akita, S., Einaga, Y., andFujita,. H., 1976, Solution properties of Poly(D-p-hydroxybutyrate). . Biosynthesis and characterization. Macromolecules 9 774... 

Lee lY, Kim MK, Chang HN, Park YH (1995) Regulation of poly-beta-hydroxybutyrate biosynthesis by nicotinamide nucleotide Alcaligenes eutmphus. FEMS Microbiol Lett 131 35—39 Lin LP, Sadoff HL (1968) Encystment and polymer production by Azotobacter vinelandii in the... 

Ward AC, Rowley BI, Dawes EA. Effect of oxygen and nitrogen limitation on poly-P-hydroxybutyrate biosynthesis in ammonium-grown Azotohacter beijerinckii.J Gen Microbiol 1977 102 61-8. 

Ascorbic acid is involved in carnitine biosynthesis. Carnitine (y-amino-P-hydroxybutyric acid, trimethylbetaine) (30) is a component of heart muscle, skeletal tissue, Uver and other tissues. It is involved in the transport of fatty acids into mitochondria, where they are oxidized to provide energy for the ceU and animal. It is synthesized in animals from lysine and methionine by two hydroxylases, both containing ferrous iron and L-ascorbic acid. Ascorbic acid donates electrons to the enzymes involved in the metabohsm of L-tyrosine, cholesterol, and histamine (128). 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

Three compounds acetoacetate, P-hydroxybutyrate, and acetone, are known as ketone bodies. They are suboxidized metabolic intermediates, chiefly those of fatty acids and of the carbon skeletons of the so-called ketogenic amino acids (leucine, isoleucine, lysine, phenylalanine, tyrosine, and tryptophan). The ketone body production, or ketogenesis, is effected in the hepatic mitochondria (in other tissues, ketogenesis is inoperative). Two pathways are possible for ketogenesis. The more active of the two is the hydroxymethyl glutarate cycle which is named after the key intermediate involved in this cycle. The other one is the deacylase cycle. In activity, this cycle is inferior to the former one. Acetyl-CoA is the starting compound for the biosynthesis of ketone bodies. 

In combination of this polymerase with purified propionyl-CoA transferase of Clostridium propionicum, a two-enzyme in vitro PHB biosynthesis system was established which allowed the PHB synthesis from (R)-hydroxybutyric acid as substrate [119]. In this way, the PHB synthesis was independent of the consumption of the expensive CoA, and hence PHA could be readily produced in a semipreparative-scale... 

Figure 3 shows parts of the in vivo metabolic route towards PHAs. The biosynthesis of poly(3-hydroxybutyric acid) (P3HB) requires the condensation of two... 

Two of the three attractant pheromones identified to date are very close structurally to those used in primary metabolism. The biosynthesis of the estolide 5 probably starts from 3-hydroxybutyric acid (4), an intermediate in fatty acid biosynthesis (Fig. 4.3). Condensation of two units furnishes the pheromone 5. The formation of cupilure (3 Fig. 4.2) can be easily explained by two methylations from ubiquitous citric acid. Both compounds are unlike any known insect pheromones, whereas the third known attractant pheromone (ketone 1 Fig. 4.1), bears some resemblance to some insect pheromones. A proper comparison of the differences and similarities between insect and arachnid pheromones will require the identification of representative compounds from most of the families of both groups of organisms. 

The biosynthesis of ketone bodies (acetoacetate, hydroxybutyrate, and acetone) occurs in the mitochondria of the liver. 

The first step in valine biosynthesis is a condensation between pyruvate and active acetaldehyde (probably hy-droxyethyl thiamine pyrophosphate) to yield a-acetolactate. The enzyme acetohydroxy acid synthase usually has a requirement for FAD, which, in contrast to most flavopro-teins, is rather loosely bound to the protein. The very same enzyme transfers the acetaldehyde group to a-ketobutyrate to yield a-aceto-a-hydroxybutyrate, an isoleucine precursor. Unlike pyruvate, the a-ketobutyrate is not a key intermediate of the central metabolic routes rather it is produced for a highly specific purpose by the action of a deaminase on L-threonine as shown in figure 21.10. 

Two molecules of acetyl CoA initially condense to form acetoacetyl CoA in a reaction which is essentially the reverse of the thiolysis step in (3-oxidation. The acetoacetyl CoA reacts with another molecule of acetyl CoA to form 3-hydroxy-3-methylglutaryl CoA (HMG CoA) (Fig. 5). This molecule is then cleaved to form acetoacetate and acetyl CoA. (HMG CoA is also the starting point for cholesterol biosynthesis see Topic K5.) The acetoacetate is then either reduced to D-3-hydroxybutyrate in the mitochondrial matrix or undergoes a slow, spontaneous decarboxylation to acetone (Fig. 5). In diabetes, acetoacetate is produced faster than it can be metabolized. Hence untreated diabetics have high levels of ketone bodies in their blood, and the smell of acetone can often be detected on their breath. 

Figure 19.10 Ketone body biosynthesis in the mitochondria. HBDH is /3-hydroxybu-tyrate dehydrogenase. (Reproduced by permission from Li PK. /3-Hydroxybutyrate. Clin Chem News February 13, 1985.)...

Masamune, S., Walsh, C. T., Sinskey, A. J., Peoples, O. P. Poly-(R)-3-hydroxybutyrate (PHB) biosynthesis mechanistic studies on the biological Claisen condensation catalyzed by -ketoacyl thiolase. PureAppl. Chem. 1989, 61,303-312. 

Mitochondrial and cytosolic biosynthesis and utilization of HMG-CoA in the liver. The molecules indicated by an asterisk are the ketone bodies. Acetoacetate and /i-hydroxybutyrate (after conversion to acetoacetate) are metabolized in extrahepatic tissues. Acetone is excreted in the lungs. Note the cytosolic multifunctional isoprenoid pathway for cholesterol biosynthesis. The double arrow indicates a multistep pathway. 

The answer is e. (Murray, pp 190—198. Scriver, pp 1521—1552. Sack, pp 121-138. Wilson, pp 287-317.) The major fate of acetoacetyl CoA formed from condensation of acetyl CoA in the liver is the formation of 3-hydroxy-3-methylglutaryl CoA (HMG CoA). Under normal postabsorp-tive conditions, HMG CoA production occurs in the cytoplasm of hepatocytes as part of the overall process of cholesterol biosynthesis. However, in fasting or starving persons, as well as in patients with uncontrolled diabetes mellitus, HMG CoA production occurs in liver mitochondria as part of ketone body synthesis. In this process, HMG CoA is cleaved by HMG CoA lyase to yield acetoacetate and acetyl CoA. The NADH-dependent enzyme P-hydroxybutyrate dehydrogenase converts most of the acetoacetate to P-hydroxybutyrate, These two ketone bodies, acetoacetate and P-hydroxybutyrate, diffuse into the blood and are transported to peripheral tissues. 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

A dramatic rise of the cost for liquid and gaseous hydrocarbons stimulates development of novel biopolymers which production is not depended on fossil fuels. Fermentative biosynthesis of poly(3-hydroxybutyrate) [PHB] and its homologues - poly(3-hydroxyalkonoates) [PHAs] bases on using renewable organic substrates. Hydrocarbons wastes of food- and wine/juice industries (sugars, melissa, starch et al.) present the basic structural material for bacterial PHB (and PHA). Utilization of hydrocarbons dining biosynthesis of PHA is favorable to eco-efficiency. 

Jossek, R., Reichelt, R., and Steinbiichel, A. (1998) In vitro biosynthesis of poly(3-hydroxybutyric acid) by using purified poly(hydroxyalkanoic acid) synthase of Chromatium vinosum. Appl. Microbiol. Biotechnol., 49, 258-266. 

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