Enzyme glycerol-3-phosphate dehydrogenase

In fact, the enzyme glycerol 3-phosphate dehydrogenase catalyzes this reaction (see Fig. 21-17). 

Glycerol-3-phosphate, thus formed, can be oxidized to dihdroxyacetone phosphate by the enzyme glycerol-3 -phosphate dehydrogenase. 

The glycerol-3-phosphate then passes through the outer mitochondrial membrane and is oxidized to dihydroxyacetone phosphate by the enzyme glycerol-3-phosphate dehydrogenase. This enzyme, which is located in the inner mitochondrial... 

Liver cells contain two different but closely related enzymes glycerol phosphate dehydrogenase which is specific for NAD, and acylglycerol phosphate dehydrogenase, which is NADP specific. Both enzymes have B stereospecificity for the pyridine nucleotide 93. They apparently have different metabolic functions. 

A specific example will illustrate some important points about the rate of displacement of a ligand with high afiinity by another with relatively lower affinity. The rate of dissociation of NADH from its complex with the enzyme glycerol phosphate dehydrogenase (E) has been investigated by the displacement of NADH by NAD". At 10 C the two reactions can be described by the following equilibrium constants ... 

GLYCEROL PHOSPHATE DEHYDROGENASE IS AN NADH OXIDIZING ENZYME RELATED TO GLYCOLYSIS 541... 

As depicted in Figure 6.8 the stability screening was based on DERA activity assay, the retro-aldol reaction of 2-deoxy-D-ribose 5-phosphate to acetaldehyde and D-glyceraldehyde 3-phosphate. D-glyceraldehyde 3-phosphate is further converted by the auxiliary enzymes triose phosphate isomerase and glycerol phosphate dehydrogenase. As the latter reaction consumes NADH it can be measured spectro-pho to metrically by the decrease in absorbance at 340 nm. 

The regulatory role of calcium ions in intermediary metabolism is well documented. Calcium has been shown to be involved in activation or inhibition of specific enzyme systems [105], For example, it activates cyclic nucleotide phosphodiesterase, phosphofructokinase, fructose 1 6 biphosphatase, glycerol phosphate dehydrogenase, pyruvate dehydrogenase phosphatase and pyruvate dehydrogenase kinase. Calcium ions inhibit pyruvate kinase, pyruvate carboxylase, Na+/K+-AT-Pase and adenylate cyclase. 

The secondary alcohol group of glycerol-3-phosphate is then oxidized by NAD to a ketone. The enzyme that catalyzes this reaction is called glycerol phosphate dehydrogenase. Recall that a dehydrogenase is an enzyme that oxidizes its... 

A coupled enzyme system can also be used. The glycer-aldehyde-3-phosphate formed is converted to dihydroxy-acetone phosphate by triosephosphate isomerase and this is followed by reduction by glycerol phosphate dehydrogenase. The oxidation of NADH in this last stage is measured spectrophotometrically. 

Figure 5.8 Stopped-flow record of NADH oxidation at 20 °C in a reaction mixture containing dihydroxyacetone phosphate (DHAP) (5 pM), a-glycerol-phosphate dehydrogenase (30 pN), NADH (50 pM), in Tris-HCl buffer (0.1 M) and EDTA (2 mM) adjusted to pH 7.5 with NaOH. One syringe contained DHAP and the other enzyme and NADH. The baseline shown is the end of the reaction. The ratio of the fast step to the slow step shows as the ratio of available substrate to precursor which is slowly converted to substrate (for detail see text and Reynolds et ai, 1971).

Many enzymes in the mitochondria, including those of the citric acid cycle and pyruvate dehydrogenase, produce NADH, aU of which can be oxidized in the electron transport chain and in the process, capture energy for ATP synthesis by oxidative phosphorylation. If NADH is produced in the cytoplasm, either the malate shuttle or the a-glycerol phosphate shuttle can transfer the electrons into the mitochondria for delivery to the ETC. Once NADH has been oxidized, the NAD can again be used by enzymes that require it. 

FADH is produced by succinate dehydrogenase in the citric acid cycle and by the a-glycerol phosphate shuttle. Both enzymes are located in the inner membrane and can reoxidize FADHj directly by transferring electrons into the ETC. Once FADH2 has been oxidized, the FAD can be made available once again for use by the enzyme. 

All these components are in the inner membrane of the mitochondria as shown in Figure I-I3-3. Succinate dehydrogenase and the a-glycerol phosphate shuttle enzymes reoxidize their FADHj and pass electrons directly to CoQ. 

Glycerol 3-phosphate can arise in two ways, either (i) from glycerol, via the enzyme glycerol kinase or (ii) from dihydroxyacetone phosphate, which is produced in glycolysis, by reduction with NADH, catalysed by glycerol-3-phosphate dehydrogenase ... 

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