1. Enzymes protein catalysts catalyst substrates products
have great specificity for the reaction catalyzed and the molecules acted on
catalyst
substance that increases the rate of a reaction without being permanently altered
substrates
reacting molecules
products
substances formed by reaction

2. Structure and Classification of Enzymes
some enzymes are composed solely of one or more polypeptides
some enzymes are composed of one or more polypeptides and nonprotein components

3. Enzyme structure apoenzyme cofactor holoenzyme = apoenzyme + cofactor
protein component of an enzyme
cofactor
nonprotein component of an enzyme
prosthetic group – firmly attached
coenzyme – loosely attached
holoenzyme = apoenzyme + cofactor

4. Coenzymes
often act as carriers, transporting substances around the cell

6. The Mechanism of Enzyme Reactions
a typical exergonic reaction
A + B  AB‡  C + D
transition-state complex –
resembles both the substrates and the products

7. activation energy – energy required to form transition-
state complex
without enzyme
with enzyme
enzyme speeds up reaction by lowering Ea

8. Interaction of enzyme and substrate
catalytic site
lock-and-key model

9. How enzymes lower Ea
by increasing concentrations of substrates at active site of enzyme
by orienting substrates properly with respect to each other in order to form the transition-state complex

10. The Effect of Environment on Enzyme Activity
enzyme activity is significantly impacted by substrate concentration, pH, and temperature

11. Effect of [substrate] rate increases as [substrate] increases
no further increase occurs after all enzyme molecules are saturated with substrate

12. Effect of pH and temperature
each enzyme has specific pH and temperature optima
denaturation
loss of enzyme’s structure and activity when temperature and pH rise too much above optima

14. Enzyme Inhibition competitive inhibitor noncompetitive inhibitor
directly competes with binding of substrate to active site
noncompetitive inhibitor
binds enzyme at site other than active site; changes enzyme’s shape so that it becomes less active
Figure 8.19 – succinate dehydrogenase is inhibited by competitive inhibitor malonic acid

15. Control of Enzyme Activity
allosteric regulation
covalent modification
feedback inhibition

16. Allosteric Regulation
enzyme inactive –
can’t bind substrate
allosteric
enzyme
effector
binding alters
shape of
active site
example of a positive effector
enzyme
catalyzes
reaction

17. An example – aspartate carbamoyltransferase (ACTase) from E. coli
functions in pyrimidine synthesis
Figure 8.24a: diagram showing 6 catalytic polypeptide chains and 6 regulatory chains; enzyme viewed from top
Figure 8.24b: less active state of ACTase viewed from side
Figure 8.24c: more active state of ACTase

18. ATP increases activity
CTP decreases activity

19. CTP, a negative effector, increases the K0
CTP, a negative effector, increases the K0.5 value while ATP, a positive effector, lowers the K0.5. The Vmax remains constant.

20. Covalent Modification of Enzymes
reversible addition or removal of a chemical group alters enzyme activity

21. Feedback Inhibition also called end product inhibition
inhibition of one or more critical enzymes in a pathway regulates entire pathway
pacemaker enzyme
catalyzes the slowest or rate-limiting reaction in the pathway

22. each end product regulates its own branch of the pathway isoenzymes –
different
enzymes that
catalyze same
reaction
each end
product
regulates the
initial
pacemaker
enzyme
Feedback inhibition of a branching pathway with two end products (P and Q). Reaction converting substrate A to intermediate B is catalyzed by isoenzymes, each regulated by a different end product (P or Q).