Question

6. Discuss and explain ways that metabolic pathways are regulated. 7. Outline the three phases of glycolysis and identify the net products. 8. Describe the citric acid cycle and list the net products of the cycle.

Answer 1

6.

Metabolic pathways are regulated as:

1. Feedback regulation-

• Enzymes of several metabolic activities are regulated by feedback regulation.
• Negative feedback results in control of the variables and bringing them back to the normal or initial ranges. The change in one direction of the regulated variables, cause a negative effect on the other direction or the initial stimulus.
• Positive feedback enhances or increases the variables to higher than normal levels as per the initial stimulus, according to the specific requirement ant a specific condition.
• Example- feedback inhibition of enzyme Threonine deaminase, by product isoleucine.

2 Allosteric regulation:

• Allosteric effectors are molecules that binds to specific site called the allosteric site, apart from the active site or site for substrate binding.
• They usually function on multi-subunit enzymes and is mediated through an effector.
• The binding of allosteric effectors regulate enzyme activity.
• The effector may either increase or decrease the enzyme activity.
• The binding of the effector causes a change in structure of the subunit, which is transferred to other sub-units. This change ultimately reaches the active site of the enzyme and affect the transition state of enzyme-substrate binding.
• Example-Pyruvate kinase (PK), is allosterically activated by accumulation of Fructose 1,6-bisphosphate.

3. Substrate concentration and removal of product:

• Decrease in substrate will decrease the amount of product produced (till enzyme saturation point). Thus, depending upon the concentration of substances the type of metabolic reaction may be conducted.
• If there is glucose starvation, pyruvate may enter the gluconeogenesis pathway. Decreased sugar level and glucagon increases during fasting. During carbohydrate starvation, the carbon-skeleton of amino acids may supplement to glucose.The process of gluconeogenesis is initiated.
• Removal of product will drive reactions in certain direction.

4. Presence in energy state as ATP/AMP

• When there is decrease in cellular energy (ATP), there in increase in AMP level. cAMP activates both PFK and PK.