UNIT 1: Biochemistry
1.7: Enzymes
pg. 50 – 57
Cellular and chemical reactions are controlled by enzymes.
Most enzymes are proteins and there about 4000 different types in a living
cell.
Enzymes are highly specific to the substrate they catalyze. To speed up the
chemical reaction, the enzyme lowers the activation energy required to break
chemical bonds.
Figure 1: A catalyst speeds up exergonic and endergonic reactions by
lowering the activation energy, but it does not change the value of ∆G.
a) The effect of a catalyst on an exergonic reaction, pg. 50.
Enzyme - is a biological catalyst, usually a protein that speeds up a chemical
reaction.
Substrate - is a substance that is recognized by and binds to an enzyme.
Enzymes and Substrates
Active site - is a pocket or groove in an enzyme that binds its substrate.
Induce-fit model - is a model of enzyme activity that describes how an
enzyme changes shape to better accommodate a substrate.
Enzymes temporarily attaches to the substrate, allows the reaction to occur,
than is released to support the reaction again.
Four Characteristics of Enzymes:
a.
b.
c.
d.
highly specific to one substrate
will support the react in both directions
will not make anything occur unless it would occur on its own
it is not consumed in the reaction
Enzymes only interact with their substrate at their active site. This groove is
specific to the shape of the substrate.
The enzyme-substrate reaction is explained by the induce-fit model. The
enzymes groove or active site conforms to the shape of the substrate. This
stresses the bonds of the substrates, enzyme-substrate complex, so the
reaction occurs, requiring less energy. After the reaction occurs the enzyme
is released to support the reaction again with new substrates.
Figure 2: The enzyme maltase catalyzes the hydrolysis of maltose into
two separate glucose molecules by breaking the α 1-4 glycosidic linkage.
Pg. 70
Cofactors and Coenzymes
Cofactor - is a non-protein group that binds to an enzyme and is essential to
catalytic activity.
Cofactors are often metals (iron, copper, zinc, and manganese).
Coenzyme - is an organic molecule that acts as a cofactor of an enzyme.
Conditions and Factors That Affect Enzyme Activity
There are factors that can alter enzyme activity, enzyme-substrate
concentration, temperature, and pH, controlling rates of reactions in the cell.
The concentration of both the enzyme and the substrate can affect the rate of
a reaction. If there is more substrate available, then the speed of reaction is
dependent on the enzyme concentration. If there is ample enzyme available
then the reaction is dependent on the concentration of the substrate.
Enzymes can reach optimal activity within a narrow range of temperature
and pH values. When temperature and pH are outside the optimal range,
enzyme activity decreases.
Figure 5: Enzymes have an optimal temperature and optimal pH levels,
pg. 52
Enzyme Inhibitors
Competitive inhibition - is a situation in which a competitor substance binds
to a normal substrate binding site to block enzyme activity.
Noncompetitive inhibition - is a situation in which molecules bind to an
enzyme at a site that is not the active site, thus blocking enzyme activity.
Figure 5: a) Normal, b) competitive, and c) non-competitive inhibition,
pg. 53
Allosteric site - is a binding site on an enzyme that binds regulatory
molecules.
Allosteric regulation - is the regulation of one site of a protein by binding to
another site on the same protein.
Figure 6:
a) changes in shape of Allosteric enzymes
b) Allosteric regulation, pg. 54
Feedback inhibition - is the regulation of a pathway by one of the products
of this pathway.
Figure 7: The diagram shows feedback inhibition in the pathway that
produces isoleucine from threonine, pg. 54
Chapter 1: Summary
pg. 52
Chapter 1: Self-Quiz
pg. 63
Chapter 1: Review
pg. 64 - 69