Biology 102

Part II

Fall 2001

R. Brundage

A.Enzymes are catalytic molecules.

1.Enzymes speed up reactions.

2.Enzymes can be reused.

3.Enzymes, at least some of them, can recognize both reactants and

products in order to catalyze a reaction in both directions.

4.Enzymes are very selective about the substrates to which they will

bind and thereby bring about change.

B.Enzyme-Substrate Interactions

1.Enzymes increase the rate of a reaction by lowering the activation energy

(the amount of energy needed to bring colliding molecules to the

transition state) through extensive bonding of substrate at the active site.

2.The active site is a crevice where the substrate binds to the enzyme during

a reaction; in Koshland’s induced-fit model, structural changes during

binding allow a more precise fit.

3.Reactants must reach a "transition" state in order for a reaction to proceed;

the active site helps this by:

a.boosting local concentrations, thus helping substrates get

together;

b.orienting substrates in positions favoring reaction;

c.promoting acid-base reactions;

d.shutting out water.

A.Enzymes and Environmental Conditions

1.Because enzymes operate best within defined temperature ranges, high

temperatures decrease reaction rate by disrupting the bonds that maintain

three-dimensional shape (denaturation occurs).

2.Most enzymes function best at a pH near 7 (pepsin in the stomach is an

exception); higher or lower values disrupt enzyme shape and halt

function.

B.Control of Enzyme Function

1.Some controls regulate the number of enzyme molecules available by

speeding up/slowing down their synthesis.

2.Allosteric enzymes have (in addition to active sites) regulatory sites where

control substances can bind to alter enzyme activity; if this control

substance is the end product in the enzyme’s metabolic pathway,

feedback inhibition occurs.

C.Enzyme Helpers

1.Cofactors are nonprotein groups that bind to many enzymes and make

them more reactive.

2.Coenzymes are large organic molecules such as NAD+, FAD, and NADP+

that transfer protons and electrons from one substrate to another.

3.Inorganic metal ions such as Fe++ also serve as cofactors when assisting

membrane cytochrome proteins in their electron transfers in chloroplasts and

mitochondria.

A.Energy is released from storage molecules (such as glucose) in controlled steps via a

series of intermediate molecules.

1.Electrons released during bond breaking are transferred stepwise through the

components of electron transport systems located on various cell membranes.

2.Each time a donor gives up an electron it is oxidized; if it gains, it is reduced.

B.Electron transport systems are similar to staircases where electrons flow down the

C.The energy is harnessed to move hydrogen ions, which in turn establish pH and

electric gradients necessary for ATP production.