Phosphoketolase

The mechanistic chemistry of the acetolactate synthase and phosphoketolase reactions (shown below) is similar to that of the transketolase reaction (Figure 23.34). Write suitable mechanisms for these reactions. 

The hypE proteins are 302-376 residues long and appear to consist of three domains. Domain 1 shows sequence identity to a domain from phosphoribosyl-aminoimida-zole synthetase which is involved in the fifth step in de novo purine biosynthesis and to a domain in thiamine phosphate kinase which is involved in the synthesis of the cofactor thiamine diphosphate (TDP). TDP is required by enzymes which cleave the bond adjacent to carbonyl groups, e.g. phosphoketolase, transketolase or pyruvate decarboxylase. Domain 2 also shows identity to a domain found in thiamine phosphate kinase. Domain 3 appears to be unique to the HypF proteins. 

PHOSPHOKETOLASE PHOSPHOLIPASE See specific enzyme MICELLE LIPASE... 

OXALYL-CoA DECARBOXYLASE PHOSPHOKETOLASE PYRUVATE DECARBOXYLASE PYRUVATE DEHYDROGENASE TRANSKETOLASE... 

Figure 13.4 The phosphoketolase pathway used by heterofermentative lactic acid bacteria. (Adapted from Stanier et al. 1970 and Gottschalk 1979.)...

A reaction that is related to that of transketolase but is likely to function via acetyl-TDP is phosphoketolase, whose action is required in the energy metabolism of some bacteria (Eq. 14-23). A product of phosphoketolase is acetyl phosphate, whose cleavage can be coupled to synthesis of ATP. Phosphoketolase presumably catalyzes an a cleavage to the thiamin-containing enamine shown in Fig. 14-3. A possible mechanism of formation of acetyl phosphate is elimination of HzO from this enamine, tautomerization to 2-acetylthiamin, and reaction of the latter with inorganic phosphate. 

A variation of the heterolactic fermentation is used by Bifidobacterium (Eq. 17-34).149 Phosphoketolase and a phosphohexoketolase, which cleaves fructose 6-P to erythrose 4-P and acetyl-P, are required, as are the enzymes of the sugar rearrangement system (Section E,3). The net yield of ATP is 2 V2 molecules per molecule of glucose. 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

Some bacteria that lack the usual aldolase produce ethanol and lactic acid in a 1 1 molar ratio via the "heterolactic fermentation." Glucose is converted to ribulose 5-phosphate via the pentose phosphate pathway enzymes. A thiamin diphosphate-dependent "phosphoketolase" cleaves xylulose 5-phosphate in the presence of inorganic phosphate to acetyl phosphate and glyceraldehyde 3-phosphate. 

Propose a mechanism for the phosphoketolase reaction and write a balanced set of equations for the fermentation. 

When screening a large number of Lactobacillus strains for lactate production using hemicellulose hydrolysate, strains L. pentosus CHCC2355 and L. brevis CHCC2097 were selected and evaluated further (Garde et al, 2002). Each strain produced lactate with 88% (for L. pentosus) and 61% of the theoretical yield (L. brevis), respectively, from wheat straw hydrolysate without visible inhibition. The operation of the phosphoketolase pathway in these... 

Perhaps the acid-tolerant, thermophilic Bacillus coagulans is the only known biocatalyst that naturally produces lactic acid from xylose via the pentose phosphate pathway, not the phosphoketolase pathway (Patel et al., 2006). Three strains, 17C5, P4-102B, and 36D1, can ferment both hexoses and pentoses to pure L(+)-lactic acid at 50 °C and pFI 5.0, an optimal environment... 

LAB are non-respiring microorganisms, principally generating ATP by fermentation of carbohydrates coupled to substrate-level phosphorylation. The two major pathways for the metabolism of hexoses are homofermentative or glycolysis (Embden-Meyerhof pathway), in which lactic acid is virtually the only end-product, and heterofermentative (phosphoketolase pathway), in which other end-products such as acetic acid, C02, and ethanol are produced in addition to lactic acid (Axelsson et al., 1989 Kandler, 1983 Zourari et al., 1992). 

Figure 4.2 Heterolactic fermentation. The fermentation of one mole of glucose yields one mole each of lactic acid, ethanol, and carbon dioxide, via the 6-phosphoglucanate/phosphoketolase pathway.

Various bacteria, and particularly the lactobadlfi, have been reported to use phosphoketolase (EC 4.1.2.9) during the metabofism of xylose [123-126]. Phosphoketolase has been reported in a few aerobic yeasts that use xylose rapidly [127-129], and the capacity of a Candida sp. to accumulate lipid has been attributed to its presence [130]. However, a role for this enzyme in ethanol production from xylose has not been estabfished. 

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