The Michaelis-Menten Equation

1. The velocity of the rxn is independent of

the [reactant]

2. A 1st order enzymatic rxn where the

[substrate] is very high, [S] is >>> Km and the velocity is constant and = to V_max

What first order kinetics show in relation to the enzyme and substrate

These kinetics show a rxn velocity that is proportional to [substrate]

Two characteristics of Km

1) It is specific for an enzyme and its tested substrate

2) It is constant and does not change with [enzyme]

A low Km value indicates this

This indicates a high affinity of the enzyme for the substate

A [substrate] below Km

The velocity of the rxn is most sensitive to changes in [substrate]

A typical curve for allosteric enzymes

A Sigmoidal curve is typical for this

Two reasons for the curve of allosteric enzymes

a) A protein/enzyme having several protein subunits

b) Cooperative binding

Two general class of enzyme inhibitors

Reversible and Irriversible inhibitors

Two characteristics of competitive inhibitors

1) it is a structural analog of a substrate

2) it competes with the substrate for the

reversible binding site to the active site

At high [S], when the E-I complex is reduced

How do the Kms of a rxn compare with and without the presence of a reversible inhibitor

The Km for the inhibited rxn is higher than the uninhibited rxn

This type of inhibiton cannot be overcome at high [S]

Noncompetitive inhibition cannot be modified by this method

Condition when noncompetitive inhibition is irreversible

This happens when the noncompetitive inhibitor binds covalently

Comparing V_maxs of uninhibited and inhibited enzyme catalyzed rxns

Vmax unihibited >> than Vmax inhibited

Comparing the Kms of uninhibited and inhibited enzyme-catalyzed rxns

The Kms of these rxns are the same

Three characteristics of noncompetitive inhibition

1) it does not compete with substrate binding site

2) The formation of the E-S substrate is unaffected

3) Product formation is not possible with the E-S-I complex

Another term for the Line-Weaver Burk Plot

A double reciprocal plot

What do the x & y intercepts represent on the Line-Weaver Burke Plot

Y-axis intercept = 1/Vmax, X-axis intercept = -1/Km

What are the x & y axes represent on the Line-Weaver Burke Plot

x-axis = 1/[S] , y-axis = 1/v₀

The kind of graph Non-competitive inhibition gives on the Line-Weaver Burke Plot

Non-competitive inhibition gives a straight line

The slope of a Line-Wever Burke Plot

slope = Km/Vmax

How do the values of Km and Vmax change in competitive inhibition

For this type of inhibition, Km is larger and Vmax stays the same

How do the values of Km and Vmax change in noncompetitive inhibition

For this type of inhibition, Km stays the same and Vmax is lower

Irreversible inhibition takes place when this happens

This takes place when the inhibitor covalently binds the enzyme

How cells can overcome irreversible inhibtion

This type of ihibition can only be overcome with the synthesis of new enzymes

Three ways irreversible inhibitors can act on an enzyme

- Covalently modifying the active site,

- Binding to cystine residues, and

- Interfering with cofactors are ways these

types of inhibitors can act

Suicide inhibition

A subtype of irreversible inhibition where the drug is a structural analogue of the substrate, binds to the the active site of the enzyme, is modified by the enzyme but then does not leave the active site as a reslt of the change