Dihydroxyacetone phosphate gluconeogenesis

Glycerol may be picked up by liver and converted to dihydroxyacetone phosphate (DHAP) for gluconeogenesis, and the fetty adds are distributed to tissues that can use them. Free fatty acids are transported through the blood in association with serum albumin. 

FIGURE 21-21 Glyceroneogenesis. The pathway is essentially an abbreviated version of gluconeogenesis, from pyruvate to dihydroxyacetone phosphate (DHAP), followed by conversion of DHAP to glycerol 3-phosphate, which is used for the synthesis of triacylglycerol. 

Fate of glycerol Glycerol that is released from triacylglycerol used almost exclusively by the liver to produce glycerol 3-phc phate, which can enter either glycolysis or gluconeogenesis oxidation to dihydroxyacetone phosphate (see p. 188). 

Figure 4.24 shows the molecular structures of compounds in the glycolytic pathway, The same compounds occur in gluconeogenesis. Generally, G-6-F and pyruvate are considered to be the first and final compounds, respectively, of glycolysis. The triose phosphate intermediate, dihydroxyacetone phosphate, is of special interest to nutritional scientists, because both dietary fructose and glycerol enter the pathway at this point. 

D. The only intermediate included on the list that glycerol has in common with lactate is glucose 6-phosphate. Glycerol enters gluconeogenesis as dihydroxyacetone phosphate. Therefore, it bypasses the other compounds. 

The remaining steps of gluconeogenesis occur in the cytosol (Pig. 31.8). Starting with PEP as a substrate, the steps of glycolysis are reversed to form glyceraldehyde 3-phosphate. Por every two molecules of glyceraldehyde 3-phosphate that are formed, one is converted to dihydroxyacetone phosphate (DHAP). These two triose... 

Fig. 13-12 Entry of glycerol into gluconeogenesis via dihydroxyacetone phosphate.

Dihydroxyacetone (No. 1716) is classified chemically as a ketotriose. It is endogenous in animals and is readily phosphorylated in vivo to form dihydroxyacetone phosphate. Dihydroxyacetone phosphate is involved in several common metabolic pathways, glycolysis and gluconeogenesis, and the glycerophosphate shuttle (Michal, 1999). It is also part of the Calvin cycle in plant metabolism (Michal, 1999). 

Triose-phcfiphate isomerase (TIM) (EC 5.3.1.1) and phosphoglycerate kinase (PGK) (EC 2.7.2.3) are two ubiquitous enzymes in the major pathways of carbohydrate metabolism (glycolysis, gluconeogenesis, and pentose phosphate pathway), catalyzing the interconversion of dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate, and the phospho-group transfer between... 

The glycerol is phosphorylated and converted to dihydroxyacetone phosphate (see Figure 5.10), which may be used either as a metabolic fuel (in the fed state) or for gluconeogenesis (in the fasting state). 

The synthesis of glucose (gluconeogenesis), which is then stored as starch (a mixture of a-glucose polymers, vide infra) and/or cellulose (a mixture of P-glucose polymers, vide infra), occurs by condensation of 3-phosphoglyceraldehyde (glycer-aldehyde 3-phosphate) with dihydroxyacetone monophosphate (Scheme 11.13). 

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