Oxoacids formation

The reversible intercalation of various oxoacids under oxidizing conditions leads to lamellar graphite salts some of which have been known for over a century and are now particularly well characterized structurally. For example, the formation of the blue, first-stage compound with cone H2SO4 can be expressed by the idealized equation... 

The group oxidation state of +5 is too high to allow the formation of simple ionic salts even for Nb and Ta, and in lower oxidation states the higher sublimation energies of these heavier metals, coupled with their ease of oxidation, again militates against the formation of simple salts of the oxoacids. As a consequence the only simple oxoanion salts are the sulfates of vanadium in the oxidation states +3 and +2. These can be crystallized from aqueous solutions as hydrates and are both strongly... 

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

In a similar exercise with D-methionine, Findrik and Vasic-Racki used the D-AAO of Arthrobacter, and for the second-step conversion of oxoacid into L-amino acid, used L-phenylalanine dehydrogenase (L-PheDH), which has a sufficiently broad specificity to accept L-methionine and its corresponding oxoacid as substrates. Efficient quantitative conversion in this latter reaction requires recycling of the cofactor NAD into NADH, and for this the commercially available formate dehydrogenase (FDH) was used (Scheme 2). 

This use of a weaker oxidant has several consequences. First, the reaction is readily reversible. Indeed, at neutral pH and with average substrate concentrations, the equilibrium tends to lie toward amino acid formation. Second, since the oxidant is not an ubiquitous oxygen, with a discardable product, but costly NAD(P)", forming NADPH, it becomes essential in any production process to find a way to reclaim or recycle the cofactor. Third, the absence of H2O2 among the products largely removes the concern about further reaction of the oxoacid through oxidative decarboxylation. 

A major aim of amino acid catabolism is removal of the a-NH2 group, which results in the formation of ammonia which is then converted to urea. The removal of the a-NH2 group for most amino acids results in the formation of a carbon-compound, which is usually an oxoacid (e.g. the oxoacid for alanine is pyruvate). 

For many amino acids, the accepting oxoacid (i.e. oxoacida) is oxoglutarate, so that transamination results in the formation of glutamate ... 

The fate of the oxoacid is either (i) formation of a common intermediate of metabolism, i.e. an intermediate within a well-established metabolic pathway (e.g. oxaloacetate or pyruvate, in the above examples), or (ii) conversion to a common intermediate , e.g. oxoisocaproate is converted to acetyl-CoA (see Appendix 8.3). 

Figure 8.17 The metabolism of branched-chain amino acids in muscle and the fate of the nitrogen and oxoacids. The a-NH2 group is transferred to form glutamate which is then aminated to form glutamine. The ammonia required for aminab on arises from glutamate via glutamate dehydrogenase, but originally from the transamination of the branded chain amino acids. Hence, they provide both nitrogen atoms for glutamine formation.

Figure 8.23 Formation of glutamine from glucose and branched-chain amino adds in muscle and adipose tissue and probably in the lung. Oxoacids may also be released into blood for oxidation in the liver.

Some of the reactions, e.g., that of isoprene with OH and NO, were discussed earlier in this chapter. Table 6.26 summarizes some of the major products observed in the gas-phase reactions of several other biogenic hydrocarbons with OH and 03 (Atkinson, 1997a). These products are anticipated, based on the mechanisms described earlier in this chapter. As also expected, the yields of these major products generally do not account for 100% of the reactant lost, and there are a number of other products, including multifunctional species, that are also formed. As an example, the formation of more than 30 individual products has been observed from the reaction of a-pinene with O, in air, some of which are unidentified, and the same is true for the A3-carene reaction (Yu et al., 1998). Products included hydroxy oxoacids, hydroxy dicarbonyls, and dicarbonyls. The formation of low-volatility products that form particles (e.g., Hoffmann et al., 1998 Jang and Kamens, 1999) is likely responsible for a significant fraction of... 

Klaning, U. K., K. Sehested, and T. Wolff, Ozone Formation in Laser Flash Photolysis of Oxoacids and Oxoanions of Chlorine... 

The addition of an enolate anion to C02 to form a (3-oxoacid represents one of the commonest means of incorporation of C02 into organic compounds. The reverse reaction of decarboxylation is a major mechanism of biochemical formation of C02. The equilibrium constants usually favor decarboxylation but the cleavage of ATP can be coupled to drive carboxylation when it is needed, e.g., in photosynthesis. 

Decarboxylation of p-oxoacids. Beta-oxoacids such as oxaloacetic acid and acetoacetic acid are unstable, their decarboxylation being catalyzed by amines, metal ions, and other substances. Catalysis by amines depends upon Schiff base formation,232 while metal ions form chelates in which the metal assists in electron withdrawal to form an enolate anion.233 235... 

The a-oxoacid dehydrogenases yield CoA derivatives which may enter biosynthetic reactions. Alternatively, the acyl-CoA compounds may be cleaved with generation of ATP. The pyruvate formate-lyase system also operates as part of an ATP-generating system for anaerobic organisms, for example, in the "mixed acid fermentation" of enterobacteria such as E. coli (Chapter 17). These two reactions, which are compared in Fig. 15-16, constitute an important pair of processes both of which accomplish substrate-level phosphorylation. They should be compared with the previously considered examples of substrate level phosphorylation depicted in Eq. 14-23 and Fig. 15-16. 

The 2-oxoacid p-hydroxyphenylpyruvate is decar-boxylated by the action of a dioxygenase (Eq. 18-49). The product homogentisate is acted on by a second dioxygenase, as indicated in Fig. 25-5, with eventual conversion to fumarate and acetoacetate. A rare metabolic defect in formation of homogentisate leads to tyrosinemia and excretion of hawkinsin97 a compound postulated to arise from an epoxide (arene oxide) intermediate (see Eq. 18-47) which is detoxified by a glutathione transferase (Box 11-B). 

The formation of an oxoacid salt from the binary oxides can be seen as an O2- transfer reaction. For example, the equation ... 

Formation of Phosphorus Oxoacids with P-P-P-P-P-P and P-P-P-P-P Frameworks and Related Compounds... 

The aldehyde group of pyridoxal phosphate accepts the amino group from an amino acid by formation of a Schiff base (Chap. 1). In this process the amino acid is converted to a 2-oxoacid, and pyridoxal phosphate is converted to pyridoxamine phosphate. The amino group on pyridoxamine phosphate can now be transferred to another 2-oxoacid, converting it to an amino acid. In this second reaction, the pyridoxamine phosphate is converted back to pyridoxal phosphate. 

In the fed state, when there is abundant protein and carbohydrate, dietary protein is hydrolyzed to amino acids. Those amino acids not required for protein synthesis are converted to 2-oxoacids by the aminotransferases. The 2-oxoacids are then converted into lipids and carbohydrate for storage. Glutamate dehydrogenase catalyzes the formation of ammonia from the excess amino groups derived from the amino acids this ammonia is excreted as urea. 

The identity of the enzyme(s) involved in the latter reaction has been debated (13). However, the formation of the above hydro-xyketone, in analogy with acetoin, has been conceptualized as the consequence of the condensation of the "active" form of acetaldehyde, that is formed by decarboxylative addition of pyruvate to thiamine pyrophospate, with benzaldehyde.The role of pyruvate, in fact has been established. The same mechanism can be invoked for the reaction of cinnamaldehyde.lt is known that the pyruvate decarboxylase (E.C. 4.1.1.1) accepts as substrates a-oxoacids... 

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