Glyceraldehydes-3-phosphate Subject

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

The enzyme that catalyzes the conversion of PEP to pyruvate is pyruvate kinase. Liver pyruvate kinase is stimulated allosterically by fructose-1,6-diphosphate, AMP, ADP, and glyceraldehyde-3-phosphate. It is inhibited by alanine, ATP, NADH, and, more importantly, by cAMP- and Ca2 calmodulin-controlled phosphorylation. High blood glucagon levels thus inhibit the activities of both PFK II and pyruvate kinase in the liver through phosphorylation. Transcription of pyruvate kinase is also decreased by glucagon and increased by insulin. Muscle pyruvate kinase is not subject to cAMP or Ca2+ regulation. The pyruvate kinase reaction is practically irreversible. 

DHAP is a glycolysis intermediate, whereas glyceraldehyde must be reduced by a mitochondrial enzyme, glyceraldehyde dehydrogenase, to glycerol, which is then subject to action by glycerol kinase in the liver. The aldolase seems to be the principal pathway of metabolizing fructose and depends on the initial phosphorylation step catalyzed by fructokinase, which produces fructose-l-phosphate. Fructokinase is defective in an inherited disorder, essential fructosuria. Fructose-l-phosphate aldolase is deficient in the hereditary disorder fructose intolerance. 

An enzyme that has been the subject of intensive experimental and theoretical studies is triosephosphate isomerase (TIM), which catalyses the interconversion of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP), an essential step in the glycolytic pathway (Fersht 1985). The mechanism of the enzyme has been examined by QM/MM calculations which we do not describe here because it falls outside the topic ofthis review (Bash et al. 1991). However, an additional aspect of the overall mechanism is the conformational change of an 11-residue loop region (residues 166-176) which moves more than 7 A and closes over the active site when substrate binds (Joseph et al. 1990 Lolis et al. 1990). Mutagenesis experiments have... 

Triose phosphate isomerase was the subject of a series of elegant and comprehensive isotope exchange experiments, which led to the establishment of the complete free energy profile for the catalysed reaction. In the following discussion, glyceraldehyde 3-phosphate will be abbreviated to G3P, dihydroxyacetone phosphate to DHAP and the enediolate to INT. The enzyme complexes and transition states will be denoted as in Figure 6.7. Reference to one of... 

Thiol-containing enzymes are also critical targets for NO. The active-site thiol of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is subject to modification via NO-dependent reactions. This, in turn, leads to reaction with NAD, thus initiating nonenzymatic ADP-ribosylation reactions (Mohr... 

The mode of nthesis of deoxyribose is a subject of active interest at the present time. It has been shown that deoxyribose 5-phosphate can be synthesized by condensation of acetaldehyde and glyceraldehyde 3-phosphate in the presence of an aldolase from animals and bacteria (80). A pathway for deoxyribonucleotides can be formulated which employs this aldolase. The 5-phosphate ester could be converted to a l-phosphate by a mutase, the deoxypentose phosphate condensed with a base to form a nucleoside by nucleoside phosphorylase, and the nucleoside converted to a nucleotide by a kinase [Eq. (26)]. 

Cyclopentane rings can also be formed readily by treating 6-deoxy-6-nitro-D-fructose (prepared enzymatically from dihydroacetone phosphate and 3-deoxy-3-nitro-D,L-glyceraldehyde in the presence of an aldolase then phosphatase) with acetic anhydride and boron trifluoride etherate, then subjecting the mixture to chromatography on silica gel thereby affording 63 and the epimer at the indicated carbon atom. ... 

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