Question

The ATP yield for a molecule depends on where it enters glycolysis or the citric acid cycle. The yield can be compared to the net yield of glucose, 30 ATP. Determine the net ATP yield of each starting molecule when fully oxidized to CO2. Assume that glycolysis, the citric acid cycle, and oxidative phosphorylation are fully active. net ATP yield from fructose 1,6-bisphosphate: ATP molecules net ATP yield from galactose: ATP molecules net ATP yield from dihydroxyacetone phosphate: ATP molecules net ATP yield from pyruvate: ATP molecules net ATP yield from lactate: ATP molecules net ATP yield from phosphoenolpyruvate: ATP molecules

Answer 1

The yield of ATP form Glucose oxidation via glycerate phosphate shuttle (+30 ATP) is as follows: ATP yield NADH FADH2 | -1 Pathway Glycolysis: Glucose to pyruvate (cytosol) Phosphorylation of glucose Phosphorylation of fructose-6-phosphate Dephosphorylation of 2 molecules of 1,3-BPG Dephosphorylation of 2 molecules of PEP Oxidation of 2 molecules of G3P yields 2 NADH Pyruvate conversion to acetyl-CoA 2 NADH produced Citric acid cycle | 2 molecules of GTP from 2 molecules of succinyl-CoA Oxidation of 2 molecules of isocitrate, alpha- ketoglutarate and malate yield 6 NADH Oxidation of 2 molecules of succinate yields 2 FADH2 Oxidative phosphorylation 2 NADH from glycolysis yield 1.5 ATP each if NADH is oxidized by glycerol-phosphate shuttle Oxidative decarboxylation of 2 pyruvates to 2 Acetyl- CoA 2 FADH2 from each TCA cycle NADH from TCA cycle Net yield: 3 5 3 15 30 Where, NADH (Cytosolic) = 1.5 ATP, NADH (matrix) = 2.5 ATP. FADH2 = 1.5 ATP) (A) In the given case, 2 ATP molecules are not used up to phosphorylate glucose to produce fructose 1,6 bisphosphate. Therefore, the net ATP yield from Fructose-1,6-bisphosphate is 32 ATP.

(B) Galactose is converted to glucose-1-phosphate in a series of steps which involves one substrate-level phosphorylation. Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. Subsequently, it enters glycolysis. Therefore, the net ATP yield from galactose is 30 ATP. (C) Fructose 1,6 bisphosphate breaks down into glyceraldehyde -3-phosphate and dihydroxyacetone phosphate (DHAP). DHAP is isomerized to glyceraldehyde 3 phosphate. Yield of ATP and reducing equivalents: • Glycolysis = 2 ATP., 2NADH Oxidative decarboxylation = 2 NADH • TCA cycle = 3 NADH, 1 FADH2, 2 GTP Therefore, the net ATP yield from the dihydroxyacetone phosphate is (2.5 +2.5+11) 16 ATP. (D) Pyruvate undergoes oxidative decarboxylation to form acetyl- COA Therefore, ATP yields are as follows: • Glycolysis = 0 • Oxidative decarboxylation = 2 NADH (2.5 ATP) • TCA Cycle = 3 NADH, 1 FADH2, 1 GTP (10 ATP) Therefore, the net ATP yield from pyruvate is 12.5 ATP. (E) Lactate is oxidized to pyruvate by lactate dehydrogenase coupled with the reduction of NAD+. The pyruvate is transported to mitochondria where it is oxidatively decarboxylated to form acetyl-CoA. Assuming 1.5 ATP per cytosolic NADH, 1.5 ATP is generated. Then the yields are similar to that of pyruvate. (12.5 +1.5 ATP = 14 ATP). Therefore, the net ATP yield from lactate is 14 ATP. (F) Phosphoenolpyruvate (PEP) is converted to pyruvate by concomitant production of ATP by substrate-level phosphorylation. Pyruvate undergoes oxidative decarboxylation. ATP yields are as follows: • Glycolysis = 1 ATP • Oxidative decarboxylation = 1 NADH (2.5 ATP) • TCA cycle = 3 NADH, 1 FADH2, 1 GTP (10 ATP) Therefore, the net yield from phosphoenolpyruvate is 13.5ATP.