Glycolysis Oxidation of Glucose

Glucose is the main energy source for the brain, skeletal muscles, and red blood cells. 

FIGURE 18.9 In glycolysis, the six-carbon glucose molecule is degraded to yield two three-carbon pymvate molecules. A net of two ATP is produced along with two NADH. Where in the glycolysis pathway is glucose cleaved to yield two three-carbon 

In the initial reaction, a phosphate group from ATP is added to glucose to form glucose-6-phosphate and ADP. 

The glucose-6-phosphate, the aldose from reaction 1, undergoes isomerization to fructose-6-phosphate, which is a ketose. 

The hydrolysis of another ATP provides a second phosphate group, which converts fructose-6-phosphate to fructose-1,6-bisphosphate. The word bisphosphate is used to show that the two phosphate groups are on different carbons in fructose and not connected to each other. 

---

However, it should be noted that although fat oxidation provides quantitatively more ATP than does the oxidation of glucose, the rate at which ATP is generated via 13-oxidation is slower than the rate of generation by glycolysis a fact which explains why glucose is the preferred fuel during sudden bursts of muscular activity when ATP concentration need to be topped-up rapidly. 

Table 9.7 Effect of aerobic physical training on the maximum capacity for ATP generation from conversion of glycogen to lactate (glycolysis) and complete oxidation of glucose (the Krebs cycle) in the guadriceps muscle of male and female volunteers...

TABLE 16-1 Stoichiometry of Coenzyme Reduction and ATP Formation in the Aerobic Oxidation of Glucose via Glycolysis, the Pyruvate Dehydrogenase Complex Reaction, the Citric Acid Cycle, and Oxidative Phosphorylation... 

Insulin also stimulates the storage of excess fuel as fat (Fig. 23-26). In the liver, insulin activates both the oxidation of glucose 6-phosphate to pyruvate via glycolysis and the oxidation of pyruvate to acetyl-CoA. If not oxidized further for energy production, this acetyl-CoA is used for fatty acid synthesis in the liver, and the fatty acids are exported as the TAGs of plasma lipoproteins (VLDLs) to the adipose tissue. Insulin stimulates TAG synthesis in adipocytes, from fatty acids released... 

Problem 6.2 illustrates the use of equation 6.2-1 by applying it to four net reactions that represent the oxidation of glucose to carbon dioxide and water (1) the net reaction for glycolysis, (2) the net reaction catalyzed by the pyruvate dehydrogenase complex, (3) the net reaction for the citric acid cycle, and (4) the net reaction for oxidative phosphorylation. The v in equation 6.2-1 is the apparent stoichiometric number matrix for these four reactions. The net reaction is... 

Answer NADH produced in the cytosol cannot cross the inner mitochondrial membrane, but must be oxidized if glycolysis is to continue. Reducing equivalents from NADH enter the mitochondrion by way of the malate-aspartate shuttle. NADH reduces oxaloacetate to form malate and NAD+, and the malate is transported into the mitochondrion. Cytosolic oxidation of glucose can continue, and the malate is converted back to oxaloacetate and NADH in the mitochondrion (see Fig. 19-29). 

Each turn of the TCA cycle generates three molecules of NADH. In glycolysis the oxidation of glucose produces two molecules of pyruvate therefore, the complete oxidation of glucose requires two turns of the TCA cycle, to generate six molecules of NADH. Each molecule of NADH is oxidised by the mitochondrial respiratory chain to generate approximately three molecules of ATP, and so from the NADH alone, approximately 18 molecules of ATP can be formed per molecule of glucose. 

A. It is an alternative process to glycolysis for oxidation of glucose It functions to provide NADPH for reductive biosynthesis. 

FIGURE 4.16 Styles of energy metabolism — partial oxidation of glucose by the pathway of glycolysis. 

A slight adjustment of the niunber 38 is required. The transport of electrons from the cytoplasm to the mitochondria requires a small amount of energy. This energy is used to drive the malate-aspartate shuttle. It is thought that 0.5 molecules of ATP are consumed with each ten of the malate-aspartate shuttle. Hence, transport of the electrons from the two NADHs generated by glycolysis into the mitochondria requires the input of 1 ATP. The sum of the ATPs (or ATP equivalents) produced by complete oxidation of glucose to CO2 is 37. 

---