Citrate formation

The buffer constituents are also important for measuring the proper electrophoretic mobility. So far, phosphate, acetate, borate, citrate, formate, HEPES, CHES, MES, CAPS, AMPSO, TAPS, Tricine, MOPS, and ACES have been used. We reported that the influence of these ingredients on the mobility of some drugs was negligible (20). Also, Gluck Cleveland (32) corrected the mobility of solutes using the mobility of toluenesulfonic acid, whose charge,... 

S ATP -P acetate <1-18> (<8> acetate kinase/phosphotransacetylase, major role of this two-enzyme sequence is to provide acetyl coenzyme A which may participate in fatty acid synthesis, citrate formation and subsequent oxidation [1] <3> function in the metabolism of pyruvate or synthesis of acetyl-CoA coupling with phosphoacetyltransacetylase [15] <11> function in the initial activation of acetate for conversion to methane and CO2 [19] <10> key enzyme and responsible for dephosphorylation of acetyl phosphate with the concomitant production of acetate and ATP [30]) (Reversibility r <1-18> [1, 2, 5-21, 24-27, 29-33]) [1, 2, 5-21, 24-27, 29-33]... 

Nineteen different compounds (or compound classes) that are known to be rapidly assimilated by bacterioplankton have been identified as DOM photoproducts (Table I). Five of these photoproducts are formed with source DOM from both freshwater and marine environments acetaldehyde, formaldehyde, glyoxylate, pyruvate, and amino acids. Nine others have been reported only from freshwater systems (acetate, butyrate, citrate, formate, levulinate, malonate, oxalate, succinate, and dissolved carbohydrates), whereas five have been reported only from marine systems (acetone, butanal,... 

Backman and Peden [2] used ion exclusion chromatography to determine weak carboxylic acids in rain water. Citrate, formate and acetate were identified. 

Thus, the citrate synthase reaction is exergonic and proceeds in the direction of citrate formation. 

In the first control point, citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to produce citrate (AG° = -32.2 kJ mol ). Although the reaction is reversible, the equilibrium lies very much in favor of citrate formation because of the hydrolysis of a bond in the intermediate compound, citroyl-CoA (Fig. 12-4). Citroyl-CoA is bound to citrate synthase, and the hydrolysis of the thioester bond, to produce citrate and coenzyme A, is an exergonic process. Citrate synthase is inhibited by its substrates (acetyl-CoA and oxaloacetate), and its activity is affected by... 

Hence, citrate formation, anion translocation (The other way, goes malate, say)... 

Magnesium ions bind to inorganic phosphate [P,] and to citrate. The levels of these anions, respectively, are about 3 3 and 1,2 mM in the cytosol and 17.0 and 5-2 mM in the mitochondria. The association constants for Mg-P and Mg-citrate formation are small, compared with that for Mg-ATP formation (see Table 10.16), Pj and citrate might be ex peeled to only slightly impair the formation of the Mg-ATP complex in the cell,... 

If the citrate formation is realized the retardation will run in two steps as shown in Fig. 1. 

Citric acid and calcium hydroxide cause a higher rate of citrate formation than calcium carbonate does. Bigger grain sizes and higher crystallization, especially of the carbonate, lower the formation rate additionally. These differences in the formation velocity lead to a more Step 1 type retardation with calcium hydroxide... 

But these changes in properties again can be altered by changing the amount and the rate of citrate formation. 

Retey et al. (72) used this same principle in their work on the stereochemistry of citrate formation from chiral acetyl-CoA with Si-citrate synthase (Scheme 15). The chiral methyl group was converted into one of the methylene groups of succinate and the distinction between sets (I, 2, 3) and (4, 5, 6) was then based on the known different isotope effects for the removal of pro-/ ( hMd = 5.3 kH/kT = 12) vs. pro-5 (kH/kD = 1.35 kH/kj = 1.5) hydrogens of succinate in the succinate dehydrogenase reaction (73). However, the malate synthase/fumarase procedure is clearly the most commonly used method to analyze the configuration and chiral purity of chiral methyl groups. 

EXAMPLE 11.18 What is the molecular mechanism of citrate formation ... 

Long chain acyl-CoA esters inhibit the activity of isolated citrate synthase specifically (Wieland, 1968 Hsu and Powell, 1975 Caggiano and Powell, 1979) but this effect has not been demonstrated with intact mitochondria and its possible involvement in the control of acetyl-CoA utilization for citrate formation in vivo remains uncertain. Similarly an elevation of palmi-toyl-CoA generation at the outside of mitochondrial membrane in vitro increases the relative rates of ketogenesis and p-hydroxybutyrate to acetoacetate ratio, and these events can be rationalized in terms of the known inhibition of mitochondrial adenine nucleotide translocase by long chain acyl-CoA esters (Pande and Blanchaer, 1971 Shug et al., 1971) but whether this inhibition is exerted in intact cells is equivocal (Hansford,... 

NO2 > CF > citrate > formate > OH . Divalent anions were strongly retained. 

Citrate Formation. Among the condensation reactions of acetyl CoA is the condensation of two acetyl groups to form acetoacetyl CoA. This is an example of a type of reaction called thiolase. These reactions will be discussed in connection with both fatty acid oxidation (p. 144) and tryptophan degradation (Chapter VII). Quantitatively, the most important reaction that uses acetyl CoA is the one that results in citrate formation. The reaction is shown in (HI). The enz3rme that catalyzes this reaction... 

The former is called malic dehydrogenase. Although it was reported early that the enzyme is specific for DPN, in fact malic dehydrogenase from several sources has been found to react with TPN at 5-7 per cent of the rate of the DPN reaction. The affinities for DPN and TPN are quite similar, Kn equals about 10 at neutral pH. The equilibrium of the reaction at neutral pH values lies far to the side of malate and DPN, but, as discussed previously, at higher pH values the equilibrium of reactions in which H+ participates is shifted, and near pH 10 the oxidation of malate proceeds to a considerable extent and at a good rate. Even at low pH values, however, the oxidation of malate is readily carried out when coupled with an effective system for oxidizing DPNH or removing oxalacetate (as citrate formation). 

Recent isotopic and enzymatic studies on acetoacetate and citrate formation have furthered our understanding of acetylation reactions. Because of the prominence assigned in this chapter to evidence obtained from isotopic experiments, it seemed desirable to provide, in the next section, a brief discussion on the use and significance of labeling agents, nonisotopic as well as isotopic, in the study of fat metabolism. 

Acetyl CoA can enter into acetylation reactions either through the carboxyl carbon, as in the formation of acetylsulfanilamide (a so-called head condensation), or through the methyl carbon, as in citrate formation (a so-called tail condensation). The formation of acetoacetate in a mixed bacterial-pigeon liver system involves a head and tail condensation of two molecules of acetyl CoA. In this system, acetoacetate is formed from acetyl phosphate, a bacterial metabolite with a high-energy bond of AF = about 15,000 cal. per mole, by the following reactions ... 

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