Question

10 rapid exam questions

1. difference between t and r states?
2. why are allosteric curves sigmoidal?
3. what is feedback inhibition?
4. what enzyme is inhibited by ctp?
5. what does atp signal in atcase?
6. mwc vs koshland?
7. what is cooperativity?
8. role of regulatory subunits?
9. phosphorylation vs cleavage?
10. why regulate first committed step?

Explanation:

1. T (tense) state: low substrate affinity, inactive; R (relaxed) state: high substrate affinity, active, seen in allosteric enzymes.
2. Sigmoidal curves arise from cooperative substrate binding—binding one substrate increases affinity for subsequent substrates, creating a steep response.
3. Feedback inhibition: a metabolic pathway's end product inhibits an early enzyme in the pathway to prevent overproduction.
4. CTP inhibits aspartate transcarbamoylase (ATCase), a key enzyme in pyrimidine synthesis.
5. ATP signals high energy/abundant purines to ATCase, acting as an activator to promote pyrimidine synthesis (balance purine/pyrimidine levels).
6. MWC (concerted model): all subunits switch between T/R states simultaneously; Koshland (sequential model): subunits change state one by one as substrate binds, inducing conformational changes.
7. Cooperativity: substrate binding to one enzyme subunit alters affinity of other subunits for substrate, amplifying enzyme response to substrate concentration.
8. Regulatory subunits bind allosteric effectors (activators/inhibitors) to modulate the catalytic activity of the enzyme's catalytic subunits.
9. Phosphorylation: reversible addition of a phosphate group to regulate enzyme activity; cleavage: irreversible proteolytic cutting of an enzyme to activate it (e.g., zymogens).
10. Regulating the first committed step prevents wasted energy/resources on synthesizing unneeded downstream products, controlling the entire pathway efficiently.

Answer:

1. T state: low substrate affinity, inactive; R state: high substrate affinity, active.
2. Due to cooperative substrate binding.
3. End product of a pathway inhibits an early pathway enzyme.
4. Aspartate transcarbamoylase (ATCase)
5. High cellular energy/abundant purines, acts as an activator.
6. MWC: all subunits switch T/R together; Koshland: sequential subunit conformational change.
7. Subunit binding alters other subunits' substrate affinity.
8. Bind effectors to modulate catalytic subunit activity.
9. Phosphorylation: reversible; cleavage: irreversible activation via proteolysis.
10. Prevents wasteful synthesis of unneeded pathway products.