enzyme and energy

1. Chapter 4
Enzymes and Energy

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Enzymes
 Biological catalysts.
 Increase rate of chemical reactions.
 Most enzymes are proteins with
diverse structure.

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Biological Catalyst
 Chemical that:
 Increases the rate of a reaction.
 Is not changed at the end of the reaction.
 Does not change the nature of the reaction
or final result.
 Lowers the activation energy required.
 Activation energy:
 Amount of energy required for a reaction to
proceed.

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Activation Energy

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Mechanism of Enzyme
Action
 Each type of enzyme has has a
characteristic 3-dimensional shape
(conformation).
 Has ridges, grooves, pockets lined with
specific amino acids.
 Pockets active in catalyzing a reaction
are called the active sites of the
enzyme.

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Mechanism of Enzyme
Action
 Lock-and-key model of enzyme activity:
 Reactant molecules (substrates) have
specific shapes to fit into the active sites.
 Substrate fits into active sites in enzyme.
 Enzyme-substrate complex dissociates.
 Products of reaction formed and free enzyme.

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Naming of Enzymes
 Enzyme name ends with ase.
 Classes of enzymes named
according to activity.
 Enzymes that have the same
activity in different organs may
make different models called
isoenzymes.

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Control of Enzyme Activity
 Rate of enzyme-catalyzed
reactions measured by the rate
substrates are converted to
products.

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Control of Enzyme Activity
 Factors influencing rate:
 Temperature
 pH
 [cofactors and coenzyme]
 [enzyme and substrate]
 Stimulatory and inhibitory effects of
products

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Effect of Temperature
 Increase in
temperature
increases rate of
reaction.
 At body
temperature,
plateaus.
 Denature at high
temperatures.

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pH
 Each enzyme exhibits
peak activity at
narrow pH range (pH
optimum).
 Optimum pH reflects
the pH of the body
fluid in which the
enzyme is found.

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Cofactors and Coenzymes
 Cofactor:
 Attachment of cofactor causes a
conformational change in enzyme.
 Participate in temporary bonds between
enzyme and substrate.
 Coenzymes:
 Cofactors that are organic molecules
derived from niacin, riboflavin and other
H20 soluble vitamins.

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Substrate Concentration
 Rate of product
formation will
increase as the
[substrate].
 Plateau of
maximum
velocity occurs
when enzyme is
saturated.

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Reversible Reactions
 H20 + C02 H2C03
 Direction of reversible reaction
depends on the concentration of
molecules to the left and right of the
arrows.
ca

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Sequence of enzymatic reactions that begins with
initial substrate, progresses through intermediates
and end with a final product.

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An intermediate can serve as substrate for 2
different enzymes, producing 2 different products.

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Negative feedback inhibition. One of the final
products inhibits the activity of the branch enzyme.
Prevents final product accumulation.

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Inborn Errors of
Metabolism
 Inherited defect in a gene.
 Quantity of intermediates formed
prior to the defect increases.
 Final product decreases, producing
a deficiency.

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Cannot produce tyrosine, PKU results.
Cannot produce melanin, albinism results.

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Bioenergetics
 Flow of energy in living systems.
 1st
law of thermodynamics:
 Energy can be transformed, but it cannot be
created or destroyed.
 2nd
law of thermodynamics:
 Energy transformations increase entropy.
 Free energy can be used to do work.

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Endergonic Reactions
 Chemical reactions that require an
input of energy.
 Products must contain more free
energy than reactants.

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Exergonic Reactions
 Convert molecules with more free
energy to molecules with less.
 Release energy in the form of heat.
 Heat is measured in calories.
 Calorie:
 Amount of heat to raise the temperature of
one cubic centimeter of H20 one degree
Celsius.

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Coupled Reactions: ATP
 Cells cannot use heat for energy.
 Require energy released in
exergonic reactions (ATP) to be
directly transferred to chemical-bond
energy in the products of
endergonic reactions.

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Universal energy carrier of the cell.

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Oxidation-Reduction
 Reduced:
 Molecule/atom gains electrons.
 Reducing agent:
 Molecule/atom that donates electrons.
 Oxidized:
 Molecule/atom loses electrons.
 Oxidizing agent:
 Molecule/atom that accepts electrons.
 May involve the transfer of H+
rather than free
electrons.

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Reduced form.

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Oxidized form.