Lecture 11 Flashcards
What are the 3 interpretations of the Michaelis - Menten curve
– at very low S, S «< Km, then Vo = Vmax [S]/ Km
– at S = Km, Vo = Vmax/2
– At very high [S], [S]»_space;> Km, then Vo = Vmax and rate is independent of [S]
How does one determine Km and Vmax accurately ?
– X-intercept: -1/Km
Y- intercept: 1/V max
Slope: Km/Vmax
T or F, Km is a concentration value and is always positive.
True
What are the 2 types of inhibition?
– Reversible (on-off binding, noncovalent)
– Irreversible (very tight binding to enzyme, covalent)
Amongst reversible inhibitors what are the 3 modes of action? And there effect on Km and Vmax
- Competitive: This increases Km but no effects on V max
- Uncompetitive: Decrease both Km and Vmax
- Mixed: Decrease Vmax but increases Km (no competition = no change in Km)
Describe competitive inhibition
– Enzyme binds Substrate or Inhibitor (not both)
– Enzyme can be freed from I by increasing S
- Km is increased (need more S to reach Vmax/2)
- V max is unchanged
What is Ki?
It is the dissociation constant of the inhibitor
What is an example of a competitive inhibitor drug?
– the inhibitor of angiotensisn converting enzyme (ACE)
– used in treatment of hypertension
– competitive inhibitor, captopril, bound in active site and to Zn2+
– binds more tightly than angiotension I
– structural analog to angiotension I, normal substrate, which is cleaved by enzyme and released angiotension ii which then elevates BP
Describe noncompetitive inhibition
– I binds to ES complex only
– I binding possible only after S binding
– ESI (enzyme substrate inhibitor) complex cannot make P
– Vmax cannot be attained and is lower
– Km also lower (proportionally to Vmax), need less S to reach Vmax/2
– High [S] does not overcome inhibition
Describe Mixed inhibition.
– Enzyme can bind inhibitor and Substrate at the same time
– ESI complex cannot make P
– I must dissociate for catalysis to occur
– Vmax is lower
– Km is increased or same (non-comp)
– High S does not overcome inhibition
Describe Irreversibile inhibition; and what is an example?
– combine covalently with the enzymes to inactivate them
– many are transition state analogs –> meaning similar to transition state in structure
– binds very strongly to enzyme (tighter binding than substrate)
Ex: DFP binds to active site serine of enzyme, acetylcholinesterase, which is necessary for nerve conduction and causes paralysis of vital functions, very toxic substance
T or F, regulation is important for enzyme activity because it doesn’t work to have all enzymes operating at peak activity all the time
True
What are the 3 ways Enzyme activity is regulated?
– Substrate level control
– Feedback control
– Allosteric Enzymes
Describe Substrate Level Control.
– direction interaction of substrate and products with enzyme
– high levels of substrate can increase reaction rate
– however, high levels of product also compete for binding to enzyme and inhibit formation of more product
T or F, Enzymes that are subjected to tight regulation often catalyze “committed steps” along specific metabolic pathway
True; committed step is basically irreversible
Describe feedback regulation.
– Enzymes that are subjected to tight regulation often catalyze “committed steps” along specific metabolic pathway
–> committed catalytic step is basically irreversible
–> product of a committed step will essentially continue down pathway to final product
–> targeting first committed catalytic step along a enzymatic pathway helps regulate end product formation without unnecessary loss of energy
Describe Allosteric Enzymes.
– Enzymes regulated by allosteric control (allosteric enzymes) are multi-subunit proteins
– Regulation of catalytic (Kcat) and binding activities generally done by inducing significant conformational changes effected by:
–> substate itself at catalytic sites (cooperativity between multiple catalytic subunits); Homoallosteric effect
–> By other ligands at sites distinct from catalytic site to increase or decrease enzymatic activity. Heteroallosteric effect.
T or F, Allosteric controls often involved in rapid switching between a highly active catalytic state (R) and a less active state (T) of enzyme
True; useful for quick responses to rapid metabolic changes and needs
Describe Homoallosteric effects.
– a single-site M-M enzyme will have a hyperbolic curve, where as multisite binding enzyme as a sigmoidal activity curve
– any enzyme that binds cooperatively will behave at low concentrations as if it binds poorly; then when S increases and more substrate binds, the enzyme is more effective because binding is better
– then enzyme undergoes a transition from lower affinity state (T state) to higher affinity state (R)
T or F; with an enzyme that binds cooperatively, it is slow to being but over time speed increases
True
T or F; Allosteric enzymes do not follow Michaelis- Menten kinetics
True; curve is different
Describe cooperative binding and the differences in substrate concentration.
– cooperative binding: binding of 1 substrate lead to increased binding affinity for substrate and higher catalytic activity at other active sites on an allosteric enzyme
– High [S] : High binding affinity for substrate and high catalytic rates
– INcrease in [S] leader to a sharp increase in catalytic rate (high slope) until maximum rate is reached
– Low [S]: low binding affinity for substrate and low catalytic rate –> has to wait for substrate to bind
T or F, heteroallosteric effectors regulate transtition from T to R state at the same site as active site
False; regulate transition at a different site than active site (hetero)
T or F, allosteric effectors are only inhibitors of enzymes.
False, they are also activators
Describe allosteric enzyme reaction rates with modulators.
– increase or decrease the binding affinity of enzyme for its substrate
– an enzyme that exists in two conformation state (T or R), can have it’s kinetics controlled by another molecule that shifts the equilibrium from one to the other state
– Inhibitors favor T state, and Activators favor R state
T or F, Inhibitors favor R state, and Activators favor T state
– False, it is the other way around
How is ATCase an example of an allosteric enzyme?
– allosteric enzyme involved in a committed step (step#2) of a multistep enzymatic pathway leading to biosynthesis of pyrimidines nucleotides (building block of nucleic acid + rile in enzyme activity)
How is ATCase regulated by ATP and CTP?
– in terms of kinetics, CTP makes it more difficult for bound substrates to convert ATCase to R state and thus makes it more difficult to produce more product
– if CTP is high, we don’t need more produced through pyridimine biosynthesis
– on other hand, hight ATP signals a purine-rich state, which signals a need for pyrimidine biosynthesis (CTP)
– an energy-rich state, where a cell can devote energy to nucleotide biosynthesis
What is the structure of ACTase?
- has 12 subunits
- 6 catalytic chains organized in 2 trimers (3 active sites/trimer)
- 6 regulatory chains organized in 3 dimers (where heteroallosteric molecules bind)
– many contacts between regulatory dimer and catalytic trimers
Describe Feedback inhibition by CTP
– CTP acts by modulating the equilibrium between ATCase T and R states
–> CTp binds to regulatory subunits preferentially in T state
–> CTP stabilizes low catalytic efficiency T state
–> CTP shifts the equilibrium toward T state
–> CTP decreases the affinity of ATCase for substrates carbamoyl phosphate and aspartate
Understanding ATCase catalysis from its structure
– binding of substrate to catalytic sites of ATCase = large conformation changes of ATCase –> not permanent attachments
– T state of ATCase: “tense” compact form with low catalytic activity
– R state of ATCase: “relaxed” expanded form with high catalytic activity
– Substrate changes equilibrium between T and R states toward R state
Describe the different cooperative effects (Low, High)
Low [S] = Tstate»_space;> Rstate, lower binding affinity for S, low activity
High [S] = Rstate»_space;> T state, higher binding affinity for S, high activity
– Steep rate increase within narrow changes in [S] due to conformational change
