The Behavior of Proteins: Enzymes

Chapter 6

Enzymes are effective biological catalysts Enzyme: a biological catalyst that can speed

up the rate of a chemical reaction

Can increase the rate of a reaction by a factor of up to 1020 over uncatalyzed reactions

RNAs (ribozymes)

Globular proteins

Thermodynamics and Kinetics of Reaction

Thermodynamics – whether a reaction is spontaneous or not

Kinetics – determines how fast a reaction occurs

What is Activation Energy? The difference between

energies of reactants and products – Standard free energy = ΔºG

The rate of a reaction depends on its activation energy = ΔºG+

Speed up reactions - Do not alter free energy

change- Lower the activation energy

What is Activation Energy?

Energy required to initiate a reaction

ΔºG for an uncatalyzed reaction is higher than that of catalyzed reaction

Enzyme Catalysis

Consider the reaction

H2O2 H2O + O2

Temperature dependence of catalysis

• Temperature can also catalyze reaction

• Increasing temperature will eventually lead to protein denaturation

Kinetic equations of enzymatic reactions For any reaction

A + B P

The rate of reaction is given by rate equation

Rate = [A]t

[B]t

[P]t

_ _= =

Rate = k[A]f[B]g

Where k is a proportionality constant called the specific rate constantspecific rate constant

How Enzymes bind to Substrate?

In an enzyme-catalyzed reactionSubstrateSubstrate, S:, S: a reactantActive siteActive site:: the small portion of the

enzyme surface where the substrate(s) becomes bound by noncovalent forces, e.g., hydrogen bonding, electrostatic attractions, Van der Waals attractions

E + S ESenzyme-substrate

complex

What are the two Binding Models? Two models have been developed to describe

formation of the enzyme-substrate complex

Lock-and-key modelLock-and-key model:: Substrate binds to that portion of the enzyme with a complementary shape

Induced fit model:Induced fit model: Binding of the substrate induces a change in the conformation of the enzyme that results in a complementary fit

2 Models of E-S Complex Formation

Formation of Product – Figure 6.5

Chymostrypsin - An Example of Enzyme Catalysis Chymotrypsin catalyzes

The selective hydrolysis of peptide bonds where the carboxyl is contributed by Phe and Tyr

It also catalyzes hydrolysis of the ester bonds

Non-Allosteric Enzyme Behavior

• Point at which the rate of reaction does not change

• Enzyme is saturated

• Maximum rate of reaction is reached

• Hyperbolic

Allosteric Enzyme Behavior

• Sigmoidal shape- characteristic of allosterism

Michaelis-Menten Kinetics

Initial rate of an enzyme-catalyzed reaction versus substrate concentration

What is Vmax and KM?

Vmax – describes velocity of an enzyme-catalyzed reaction when there is a saturating level of substrate

- Determines the individual rate constant (Kp)

- KM is equal to substrate concentration that generates half of Vmax.

Lineweaver-Burk Plot

KM is the dissociation constant for ES; the greater the value of KM, the less tightly S is bound to E

Vmax is the turnover number

Turnover Numbers

• Vmax is related to the turnover number of enzyme:also called kcat

• Number of moles of substrate that react to form product per mole of enzyme per unit of time

V max

[ET ]

turnover _ number kcat

What is an enzyme inhibitor?

A substance that interferes with action of an enzyme and slows rate of reaction – Enzyme inhibitor

Reversible Enzyme Inhibition

Reversible inhibitorReversible inhibitor:: a substance that binds to an enzyme to inhibit it, but can be released Competitive Competitive

inhibitorinhibitor:: binds to the active (catalytic) site and blocks access to it by substrate

Reversible Enzyme Inhibition

Noncompetitive Noncompetitive inhibitorinhibitor:: binds to a site other than the active site; inhibits the enzyme by changing its conformation

Non-reversible enzyme inhibition Irreversible inhibitorIrreversible inhibitor:: a substance that

causes inhibition that cannot be reversedusually involves formation or breaking of

covalent bonds to or on the enzyme

Other types of Inhibition

Uncompetitive- inhibitor can bind to the ES complex but not to free E. Vmax decreases and KM decreases.

Mixed- Similar to noncompetitively, but binding of I affects binding of S and vice versa

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