Enzymes phosphoketolase

Pentoses and gluconate were not fermented by this pathway because of lack of enzyme phosphoketolase. This type of fermentation includes some species of the genus Lactobacillus. The important LA producers in this genus are L. acidophilus and L. delbrueckii (Hofvendahl and Hahn-Hagerdal 2000 Kwon et al. 2001). 

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... 

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. 

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. 

Fig. 1-4A). Heterofermenters, however, lack the enzyme fructose-diphosphate aldolase and must divert the flow of carbon through the 6-phospho-gluconate pathway (pentose phosphate or phosphoketolase pathway) as depicted in Fig. l-4b to yield lactic acid as well as ethanol, acetic acid (depending on redox potential), and CO2. Energetically, the consequence of only half of the carbon returning to the EMP is formation of 1 mole of ATP/glucose.

Thiamine has a physiological function in the form of thiamine pyrophosphate. This compound is the coenzyme of a number of enzymes involved in carbohydrate metabolism carboxylase, pyruvic dehydrogenase, -ketoglutaric dehydrogenase, transketolase, phosphoketolase. Thiamine monophosphate and thiamine triphosphate cannot replace the pyrophosphate ester in this function a supposed activity of the triphosphate was later shown to be due to its partial hydrolysis to the pyrophosphate . 

The xylulose 5-P phosphoketolase is the key enzyme of this pathway it catalyzes the cleavage of the pentulose 5-P molecule into acetyl-P... 

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