Enzymology - Tutorial 3: Enzyme Regulation, Electron Transport Chain (ETC) and Oxidative Phosphorylation

What are the characteristics of an allosteric enzyme?

• An enzyme that changes its conformational ensemble upon binding of an effector

• binding of an effector results in change in binding affinity at different ligand binding site

• follow sigmoidal curve

• usually composed of multiple subunits

What happens when a homotropic effector binds?

binding of the first substrate enhances the probability of second substrate binding

What happens when a heterotropic effector binds?

• effector molecule binds to site on regulatory subunit which sends a message to the catalytic subunit

• substrate binds more or less depending on whether the effector is positive or negative

What is a zymogen?

in active form/precursor of enzyme

How is a zymogen activated?

activated by the removal of a polypeptide unit

What is the process of breaking down nutrients to release energy to change ADP to ATP?

Catabolism

What is the process of changing simpler components to more complex ones with ATP?

Anabolism

What’s the difference between ΔG and ΔG^o?

• ΔG = change in free energy

• ΔG^o = change in free energy under standard conditions

  • all reactants and products are at an initial concentration of 1.0 M

  • pressure of 1.0 atm

  • temperature of 25 C

What happens if a reaction has negative ΔG?

• favorable (spontaneous) reaction

• exergonic reaction

What happens if a reaction has positive ΔG?

• unfavorable (non-spontaneous) reaction

• endergonic

What happens if a reactions ΔG = 0?

reaction is at an equilibrium

Which ETC inhibitors inhibit complex I?

rotenone

Which ETC inhibitors inhibit complex III?

antimycin A

Which ETC inhibitors inhibit complex IV?

cyanid and carbon monoxide

What prevents the influx of protons through ATP synthase?

oligomycin

What is the function of dinitrophenol?

a chemical uncoupler, uncouples ETC from OX. Phosphorylation

What happens when an allosteric activator stabilizes the enzyme in its high affinity form?

enzyme activity

What happens when an enzyme subject to allosteric activation is in its uncomplexed form?

• allosteric activation is less active

• uncomplexed form has low affinity for the substrate

What happens when an enzyme subject to allosteric inhibition is in its uncomplexed form?

• allosteric inhibition is active

• uncomplexed form has high affinity for the substrate

What happens when an allosteric inhibitor stabilizes the enzyme in its low affinity form?

little or no activity

What is the process of creating ATP from ADP?

phosphorylation

Which enzyme helps turn ATP to ADP?

kinase

What is the function of phosphatase?

• removes a phosphate group

• opposite of kinase

How are enzymes regulated by covalent modification?

ubiquitination

How are enzymes regulated by protein-protein interactions?

What is the induction and repression of enzyme synthesis?

• induction: increased synthesis of an enzyme

• repression: reduced synthesis of an enzyme

used for enzymes needed under special physiological conditions

What are the properties of isoenzymes?

• enzymes with different molecular forms\

• have similar amino acid sequences but not identical

• may differ in kinetics (K_m and V_max)

• may have different regulators

• may differ in coenzyme preference

• may have different cellular distribution

• may have different development distribution

What are the isoenzymes of creatine kinase (CK)?

How are enzymes used in clinical diagnosis?

increased plasma level of certain enzymes can be used for setting a clinical diagnosis

How does the ETC work?

• Complex I accepts electrons from NADH, and passes them to coenzyme Q (CoQ) which also receives electrons from complex II

• CoQ passes electrons to complex III, which passes them to cytochrome c (cyt c)

• Cyt c passes electrons to Complex IV, which uses the electrons and hydrogen ions to reduce molecular oxygen to water

What is the most important factor in the regulation of ETC?

ADP, referred to as respiratory control