Enzyme Inhibition by Glucose-6-Phosphate
Hexokinases are enzymes crucial for glycolysis, catalyzing the phosphorylation of glucose. Many hexokinase isoenzymes are subject to feedback inhibition by their product, glucose-6-phosphate. This mechanism helps regulate the rate of glycolysis based on cellular energy needs.
Hexokinase Isoenzymes and Inhibition
Hexokinases I, II, and III are typically inhibited by physiological concentrations of glucose-6-phosphate. This inhibition occurs when glucose-6-phosphate levels rise, signaling that the cell's energy demands are met or downstream pathways are saturated.
- Hexokinase I: Inhibited by glucose-6-phosphate.
- Hexokinase II: Inhibited by glucose-6-phosphate.
- Hexokinase III: Inhibited by glucose-6-phosphate.
Hexokinase IV (Glucokinase) Exception
Hexokinase IV, also known as Glucokinase, exhibits different regulatory properties compared to the other isoenzymes.
- Hexokinase IV is characteristically NOT inhibited by glucose-6-phosphate.
- This lack of inhibition allows liver and pancreatic cells to respond dynamically to blood glucose levels, even when glucose-6-phosphate is abundant. Glucokinase's activity is primarily regulated by glucose concentration itself and interactions with regulatory proteins.
Therefore, Hexokinase IV is the enzyme among the options that is not inhibited by glucose-6-phosphate.
Important Questions from Metabolism Glycolysis Nucleotide
- The oxidative phase of the hexose monophosphate pathway generates ________ moles of NADPH per mole of glucose 6-phosphate (answer in integer).
- 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 ________.
