Enzymes: Structure & Function

Lab #3- Enzyme Catalysis w/Catalase

Three Parts to the lab:

• Establish Baseline Amount of H2O2

• Uncatalyzed Decomposition of H2O2

• Time Trials w/Catalase to determine Rxn rate

• Procedure: – 10 ml H2O2 in a beaker

– 1.0 ml (H2O or Catalase)

– 10 ml 1 M H2SO4

– Mix well

– Take a 5 ml sample and titrate in KMnO4

– Read Initial and final measurements on buret

– Record Data

• For a chemical reaction to begin, reactants

must absorb some energy

– This energy is called the energy of activation

(EA)

– This represents the energy barrier that prevents

molecules from breaking down spontaneously

Enzymes speed up the cell’s chemical

reactions by lowering energy barriers

Enzymes

• Catalytic proteins: change

the rate of reactions w/o

being consumed

• Free E of activation

(activation E): the E

required to break bonds

• Substrate: enzyme reactant

• Active site: pocket or

groove on enzyme that

binds to substrate

• Induced fit model

• A protein catalyst called an enzyme can

decrease the energy barrier

EA

barrier

Reactants

1 Products 2

En

zym

e

Figure 5.5A

• Enzymes are selective

– This selectivity determines which chemical

reactions occur in a cell

A Specific Enzyme Catalyzes

each Cellular Reaction

How an Enzyme Works-Sucrase

How Enzymes Work

• http://www.ekcsk12.org/science/aplabrevie

w/lab02.htm

**Description of Enzyme Lab

• Lab Simulation: http://bioweb.wku.edu/courses/Biol114/enzy

me/enzyme1.asp

Effects on Enzyme Activity

• Temperature

• pH

• Substrate conc.

• Enzyme conc.

• Cofactors:

inorganic, nonprotein

helpers; ex.: zinc, iron,

copper

• Coenzymes:

organic helpers

ex. vitamins

Effects on Enzyme Activity

• pH - based on relative number of H+

• Large number of H+ ions can bond with negative charges

on the enzyme or substrate

• Proper charge matching is unable to occur

• Similar scenario occurs with OH- ion

• Numerous extra + and – charges result in the enzyme

losing its shape

Effects on Enzyme Activity

• Temperature – kinetic energy

• Molecules moving faster are more often to

collide with greater energy

• High temperatures can cause intramolecular

bonds to stress

Effects on Enzyme Activity

• Substrate Concentration – activity increases with

concentration

• Increased molecular collisions

• Enzymes have a maximum rate at which they

can work

Effects on Enzyme Activity

• Enzyme Concentration – activity increases with

concentration

• Increased molecular collisions

• Only so much substrate for enzymes to work on

Enzyme Inhibitors

• Irreversible (covalent); reversible (weak bonds)

• Competitive: competes for active site (reversible); mimics

the substrate

• Noncompetitive: bind to another part of enzyme (allosteric

site) altering its conformation (shape); poisons, antibiotics

• Inhibitors interfere with enzymes

– A competitive

inhibitor takes

the place of a

substrate in the

active site

– A noncompetitive

inhibitor alters an

enzyme’s function

by changing its shape

Enzyme inhibitors block enzyme

action

Substrate

Enzyme

Active

site

NORMAL BINDING OF SUBSTRATE

Competitive

inhibitor

Noncompetitive

inhibitor

ENZYME INHIBITION

Figure 5.8

Competitive & Noncompetitive Inhibitors

• Enzyme activity is influenced by

– temperature

– salt concentration

– pH

• Some enzymes require non-protein

cofactors such as: Fe, Zn, Cu, etc.

The Cellular environment affects

enzyme activity

• Certain pesticides are toxic to insects

because they inhibit key enzymes in the

nervous system

• Many antibiotics inhibit enzymes that are

essential to the survival of disease-causing

bacteria

– Penicillin inhibits an enzyme that bacteria use in

making cell walls

Some Pesticides and Antibiotics

inhibit Enzymes

Allosteric Enzymes-How they Work

Feedback Inhibition