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Enzymes

Why Are Enzymes So Important?

Why are we devoting one whole lecture topic to a

protein molecule?– All chemical reactions in

living organisms require enzymes to work

Image: Jumping rope, Meagan E. Klein

From the Virtual Cell Biology Classroom on ScienceProfOnline.com

Chemical Reactions• A process that changes one set of

chemicals to another set of chemicals• Reactants – elements or compounds

that enter into a chemical reaction• Products – elements or compounds

produced by a chemical reaction

Chemical Reactions

2H2 + O2 → 2H2O

reactants productsInvolves the breaking of bonds in the reactants and the formation of new

bonds in the product

Chemical equations – the symbols and numbers used to represent each element or substance in a chemical

reaction

Energy in Reactions – Energy Changes

• Chemical reactions that release energy often occur spontaneously. –Ex. 2H2 + O2 → 2H2O

• Chemical reactions that absorb energy will not occur without a source of energy.–Ex. When water is changed into

hydrogen and oxygen gas

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Types of Reactions• Exothermic-When a reaction produces heat

• Endothermic-When a reaction requires heat to “get started”

Energy in Reactions – Activation Energy

• Activation energy – energy needed to get a reaction started

http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/energy/Image9.gif

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What Are Enzymes?• Most enzymes

are Proteins • Act as Catalyst to

accelerate a reaction

• Not permanently changed in the process

• Make materials needed by the cell

Catalyst

• A substance that speeds up the rate of a chemical reaction by lowering the reaction’s activation energy

http://en.wikipedia.org/wiki/Image:Activation_energy.svg

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Enzymes• Are specific

for what they will catalyze

• End in –ase-Sucrase-Lactase-Maltase

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How do enzymes Work?

Enzymes work by weakening bonds which lowers activation energy

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Enzymes

FreeEnergy

Progress of the reaction

Reactants

Products

Free energy of activation

Without Enzyme

With Enzyme

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Enzyme-Substrate ComplexThe substance

(reactant) an enzyme acts on is the substrate

EnzymeSubstrate Joins

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Active Site• A active site of an enzyme

molecule binds to the substrate.

EnzymeSubstrate

Active Site

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enzyme

substrate

product

active site

Enzymes are not changed by the

reaction, they are reusable

Active site of the enzyme

Two substrates

Enzyme

The active site is like a lock

The substrates fit like a key in a lock

Enzyme

The activation energy for these substrates to bind together has been lowered by the enzyme.

Enzyme

Chemical reaction!!!

Basic Enzyme Diagram

Active site

The substrates have reacted and changed

into the product

Enzyme is unchanged

Lock & Key model• Fit between the substrate and the

active site of the enzyme is exact • Like a key fits into a lock very

precisely• The key is analogous to the

enzyme and the substrate analogous to the lock.

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Induced Fit• A change in the shape of an enzyme’s active site

• Induced by the substrate

Induced Fit Model• Enzymes can form to the shape

of its substrate.

http://en.wikipedia.org/wiki/File:Induced_fit_diagram.svg

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What Affects Enzyme Activity?

• Three factors:• Temperature

• pH• Inhibitors

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Temperature

- Effects rates of enzyme activity- Hot temps increase activity- Low temps reduce acitivity- High temps may denature

(unfold) the enzyme.

pH • Effect on rates of enzyme activity

–Changes in pH changes protein shape~ Denatures

–Most human enzymes = pH 6-8•depends on where in body•pepsin (stomach) = pH 3

• trypsin (small intestines) = pH 8

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Two examples of Enzyme Inhibitors

a. Competitive inhibitors: are chemicals that resemble an enzyme’s normal substrate and compete with it for the active site.

Enzyme

Competitive inhibitor

Substrate

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Inhibitors

b. Noncompetitive inhibitors:Inhibitors that do not enter

the active site, but bind to another part of the enzyme causing the enzyme to change its shape, which in turn alters the active site.

Enzyme

active site altered

NoncompetitiveInhibitor

Substrate

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Acids, Bases, and pH

pH scale

– Measurement system that indicates the concentration of hydrogen (H+) ions in solution• A scale of 1-14 is used to describe

pH

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7 is considered neutral

Substances with a pH of 1-6 are considered acidic

Substances with a pH of 8-14 are considered basic or alkaline

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Acids and Bases • Acid – any substance that forms hydrogen

ions (H+) in water– Acidic solutions contain higher

concentrations of H+ ions and pH lower than 7.

• Base – any substance that forms hydroxide ions (OH-) in water– Basic, or alkaline, solutions contain lower

concentrations of H+ ions and pH higher than 7.

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Buffers - prevent sudden changes in pH. (used to maintain homeostasis)

For Catalase Lab• EX for lab: Catalase is a common enzyme

found in nearly all living organisms exposed to oxygen (such as vegetables, fruit or animals). It catalyzes the decomposition of hydrogen peroxide to water and oxygen.

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