Chapter 15 Enzyme Regulation

Front 1

Controls over enzyme activity

Back 1

substrate availability
product inhibition
enzyme abundance(synthesis)
covalent modifications
allosteric regulation
modular proteins

Front 2

Substrate availability

Back 2

[S]>>Km then v=Vmax
[S]=Km then v=.5*Vmax
Km predicts in-vivo [S]

Front 3

Product inhibition

Back 3

when pdt accumulates and approaches equilibrium, rxn slows down
some enzymes inhibited by rxn pdts

Front 4

Enzyme abundance: synthesis and degradation

Back 4

genetic controls of level of expression
can be repressed due to lack of demand and induced in response to enviro stimuli

Front 5

Covalent modifications

Back 5

post-translational
P-ylation is classic i.e., can inhibit or enhance enzymes

Front 6

Interconvertible enzymes
(phosphotase or kinase)

Back 6

enzymes regulated by covalent modification.
Active: enzyme-OH
intermediate:ATP to ADP and
H2O to P
Inactive:enzyme-PO4

Front 7

Zymogens

Back 7

aka proenzymes
results in full activity when cleavage of peptide bonds occur
advantage: safe, faster pdtion of active enzymes, coagulation

Front 8

Proenzyme
(active insulin is generated by proteolytic cleavage of the proinsulin)

Back 8

synthesized in pancreas and secreted to digestive tract where it is processed and activated.

Front 9

Isozymes

Back 9

heteromeric enzymes with different combos of subunits
catalytic kinets and specificity vary

Front 10

lactate dehydrogenase (LDH)

Back 10

active muscle becomes anaerobic and produces pyruvate from glucose via glycolysis. Regenerate NAD from NADH, lactate released into blood.

Front 11

Modular Proteins

Back 11

reversible binding that controls the activity of the associated enzyme
i.e.:cAMP-dependentis inactive until it release subunits

Front 12

Allosteric regulation
(action at another site)

Back 12

regulates enzymes at key steps in metabolic pathways
feed-forward/back inhibitors
kinetics are sigmoid

Front 13

Vmax (y) vs. [S] (x)

Back 13

Hyperbolic: increase then level
sigmoid: "S" under hyperbolic

Front 14

Allosteric control
(molecular O2 binding curves of myo-gb and Hgb)

Back 14

myo:hyperbolic
hemo:sigmoid
venous at 30, arterial at 100

Front 15

Myoglobin

Back 15

(sperm whale): one polypeptide chain of 153 aa residues, with one heme and binds one O2

Front 16

Hemoglobin

Back 16

(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

Front 17

Myoglobin Molecule

Back 17

has 8 helical segments counting from the N-terminus
heme is cradled within the folded polypeptide chain

Front 18

Iron ion with hemes

Back 18

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.

Front 19

Displacement of Fe ion

Back 19

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.

Front 20

Hgb of an alpha-beta dimer

Back 20

packing contents occur when rounded edges touch. sliding contents are connected

Front 21

Deoxyhemoglobin

Back 21

subunit motion is hgb when molecule goes from deoxy to oxy. Twist molecule 15 degrees twice

Front 22

Models for Allosteric Behavior

Back 22

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

Front 23

Allosteric variations

Back 23

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

Front 24

Allosteric effectors may be positive or negative

Back 24

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

Front 25

Effects of activator

Back 25

increases R conformer shifts
more binding sites for S
decrease in cooperativity of S saturation curve

Front 26

Effects of inhibitor

Back 26

increase T conformers
lowers R, inhibits S and A association
increases cooperativity of S saturation curve

Front 27

Allosteric regulation can be K or V system

Back 27

K:Vmax constant, Km is affected by effectors
V:Km unaffected, Vmax is increased or decreased

Front 28

Km

Back 28

affinity for the substrate
[S]~Km--> is the K system
[S]>>Km--> is the V system

Front 29

K system
hyperbolic then sigmoid then an almost linear

Back 29

Vmax is top horizontal, (+)effector
B/w Vmax and (1/2) is no effector
bottom Right is negative effector

Front 30

V system
three parallel hyperbolic

Back 30

Negative
none
positive

Front 31

Hemoglobin

Back 31

O2 transport, storage protein
tetrameric
2 alphas, 2 betas

Front 32

Myoglobin

Back 32

monomeric, one aa
polypep cradles heme
Fe2+(ferrous), binds O2
O2 or CO binds as 6th to Fe

Front 33

Conformation change of mgb

Back 33

O2 binding does it
w/o O2 bound,Fe is out of heme plane
F helix moves when O2 binds

Front 34

Hgb function
must bind in lungs and release into capillaries

Back 34

when 1st O2 binds to Fe in hem, the Fe is drawn into plane
Initiates series of changes adjacent to subunits

Front 35

Hemoglobin function

Back 35

adjacent subunits affinity for O2 increases(cooperativity)