HMP Pathway: NADPH Yield in Oxidative Phase
The oxidative phase of the hexose monophosphate (HMP) pathway is crucial for generating NADPH, a key reducing agent. This phase involves irreversible reactions starting from glucose 6-phosphate.
Oxidative Phase Reactions
For each mole of glucose 6-phosphate entering the oxidative phase, two moles of NADPH are produced through the following key steps:
- Step 1: Glucose 6-phosphate is oxidized to 6-phosphogluconolactone, reducing one molecule of NADP\(^{+}\) to NADPH.
Glucose 6-phosphate + NADP\(^{+}\) \( \rightarrow \) 6-phosphogluconolactone + NADPH + H\(^{+}\) - Step 2: 6-phosphogluconolactone is converted to 6-phosphogluconate. (No NADPH produced here).
- Step 3: 6-phosphogluconate is oxidized and decarboxylated to ribulose 5-phosphate, reducing a second molecule of NADP\(^{+}\) to NADPH.
6-phosphogluconate + NADP\(^{+}\) \( \rightarrow \) Ribulose 5-phosphate + CO\(_{2}\) + NADPH + H\(^{+}\)
NADPH Generation Summary
Based on the reactions in the oxidative phase:
- One mole of NADPH is generated in the first oxidative step.
- Another mole of NADPH is generated in the third oxidative step (6-phosphogluconate dehydrogenase).
Therefore, a total of 2 moles of NADPH are generated per mole of glucose 6-phosphate processed through the oxidative phase of the HMP pathway.
Important Questions from Metabolism Glycolysis Nucleotide
- Which one of the following enzymes is NOT inhibited by glucose-6-phosphate?
- Assume that in the oxidative branch of the pentose phosphate pathway, each glucose-6-phosphate molecule generates the following products: 2 molecules of NADPH, 1 molecule of $\text{CO}_2$ and 1 molecule of ribulose-5-phosphate.
If 30 molecules of glucose-6-phosphate enter this pathway, and you randomly draw one molecule from the products of the pathway, the probability that this molecule is NADPH is __________ (round off to one decimal place). - Match the hormones in Group I with their metabolic precursor in Group II
Group I Group II P. 17-$\beta$ estradiol 1. Arachidonic acid Q. Thromboxane A2 2. Tyrosine R. Epinephrine 3. $\beta$-carotene S. Retinoic acid 4. Cholesterol Which of the following statements are TRUE for respiration?
P. The conversion of one molecule of pyruvate to three molecules of $CO_2$ generates four molecules of NADH
Q. Fructose 6-phospate is the principal substrate for glycolysis
R. The oxidation of glucose 6-phosphate to 6-phosphogluconate is the first step in the oxidative pentose phosphate pathway
S. The mitochondrial 'alternative oxidase' provides an alternative pathway for transfer of electrons from ubiquinone to oxygen utilizing proton pumping complex of the respiratory chain
- Phosphoglucose isomerase was incubated with 0.2 M of glucose 6-phosphate. On reaching equilibrium, 55% of glucose 6-phosphate was converted to fructose 6-phosphate. The equilibrium constant for this reaction is ________.
