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35 Cards in this Set

  • Front
  • Back
Controls over enzyme activity
substrate availability
product inhibition
enzyme abundance(synthesis)
covalent modifications
allosteric regulation
modular proteins
Substrate availability
[S]>>Km then v=Vmax
[S]=Km then v=.5*Vmax
Km predicts in-vivo [S]
Product inhibition
when pdt accumulates and approaches equilibrium, rxn slows down
some enzymes inhibited by rxn pdts
Enzyme abundance: synthesis and degradation
genetic controls of level of expression
can be repressed due to lack of demand and induced in response to enviro stimuli
Covalent modifications
P-ylation is classic i.e., can inhibit or enhance enzymes
Interconvertible enzymes
(phosphotase or kinase)
enzymes regulated by covalent modification.
Active: enzyme-OH
intermediate:ATP to ADP and
H2O to P
aka proenzymes
results in full activity when cleavage of peptide bonds occur
advantage: safe, faster pdtion of active enzymes, coagulation
(active insulin is generated by proteolytic cleavage of the proinsulin)
synthesized in pancreas and secreted to digestive tract where it is processed and activated.
heteromeric enzymes with different combos of subunits
catalytic kinets and specificity vary
lactate dehydrogenase (LDH)
active muscle becomes anaerobic and produces pyruvate from glucose via glycolysis. Regenerate NAD from NADH, lactate released into blood.
Modular Proteins
reversible binding that controls the activity of the associated enzyme
i.e.:cAMP-dependentis inactive until it release subunits
Allosteric regulation
(action at another site)
regulates enzymes at key steps in metabolic pathways
feed-forward/back inhibitors
kinetics are sigmoid
Vmax (y) vs. [S] (x)
Hyperbolic: increase then level
sigmoid: "S" under hyperbolic
Allosteric control
(molecular O2 binding curves of myo-gb and Hgb)
venous at 30, arterial at 100
(sperm whale): one polypeptide chain of 153 aa residues, with one heme and binds one O2
(human): four polypeptide chains, two (alpha) 141 aa residues and two (beta) of 146 residues, each polypeptide has a heme the tetramer bind four O2
Myoglobin Molecule
has 8 helical segments counting from the N-terminus
heme is cradled within the folded polypeptide chain
Iron ion with hemes
6 positions of the ion: 4 are in the same plane, other 2 are above and below. In mgb the F8 is the 5th ligand. in omgb O2 is the 6th.
Displacement of Fe ion
O2 binds on opp side of heme then Fe and pulls Fe back toward the molecule. Pull of His F8 displaces Fe from plane of the ring.
Hgb of an alpha-beta dimer
packing contents occur when rounded edges touch. sliding contents are connected
subunit motion is hgb when molecule goes from deoxy to oxy. Twist molecule 15 degrees twice
Models for Allosteric Behavior
Monad,Wyman, Changeux (MWC): allosteric proteins either relaxed (R) or taut (T)
T predominates in absence of substrate
S binds tighter to R than T
Allosteric variations
L=To/Ro, if L approaches 1 then no control
The S binds and locks the enzyme into the R form allowing the next S to be easier
Allosteric effectors may be positive or negative
protein can bind 3 ligands:
S: (+)homotropic effector that only binds to R @ s
activator:(+) heterotropic effector that binds to R @ F
inhibitor:(-) heterotropic that binds to T @ F
Effects of activator
increases R conformer shifts
more binding sites for S
decrease in cooperativity of S saturation curve
Effects of inhibitor
increase T conformers
lowers R, inhibits S and A association
increases cooperativity of S saturation curve
Allosteric regulation can be K or V system
K:Vmax constant, Km is affected by effectors
V:Km unaffected, Vmax is increased or decreased
affinity for the substrate
[S]~Km--> is the K system
[S]>>Km--> is the V system
K system
hyperbolic then sigmoid then an almost linear
Vmax is top horizontal, (+)effector
B/w Vmax and (1/2) is no effector
bottom Right is negative effector
V system
three parallel hyperbolic
O2 transport, storage protein
2 alphas, 2 betas
monomeric, one aa
polypep cradles heme
Fe2+(ferrous), binds O2
O2 or CO binds as 6th to Fe
Conformation change of mgb
O2 binding does it
w/o O2 bound,Fe is out of heme plane
F helix moves when O2 binds
Hgb function
must bind in lungs and release into capillaries
when 1st O2 binds to Fe in hem, the Fe is drawn into plane
Initiates series of changes adjacent to subunits
Hemoglobin function
adjacent subunits affinity for O2 increases(cooperativity)