Factors Influencing Enzyme Action

Substrate

Active site

Enzyme Enzyme-substratecomplex

Substrate

Active site

Enzyme Enzyme-substratecomplex

Enzyme reactions

enzyme + substrate

enzyme-substrate complex

enzyme + products

Factors that affect enzyme rates of reaction

• An enzyme’s rate of reaction is measured by the amount of substrate converted to a product per minute.

• An enzyme’s activity can be affected by:1. General environmental factors, such as

temperature and pH.2. Cofactors and coenzymes.3. Enzyme and substrate concentration which affects

the saturation of the active site.4. Inhibitors.

Optimal temperature fortypical human enzyme

Optimal temperature forenzyme of thermophilic (heat-tolerant bacteria)

Temperature (°C)

Optimal temperature for two enzymes

0 20 40 60 80 100

Rate

of r

eacti

onOptimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

pH

Optimal pH for two enzymes

0

Rate

of r

eacti

on

1 2 3 4 5 6 7 8 9 10

• What is the effect of temperature on enzyme function?

• What is the effect of pH on enzyme function?

1. Environmental Factors• Each enzyme has an optimal temperature in

which it can function.• Each enzyme has an optimal pH in which it can

function.• The wrong temperature of pH can denature an

enzyme, which means the enzyme proteins shape is destroyed.

2. Cofactors and Coenzymes• Cofactors are nonprotein inorganic enzyme

helpers.– For example, metal ions like iron in

hemoglobin hold oxygen.• Coenzymes are organic cofactors.

– For example, vitamins like niacin which serves as an electron acceptor.

Example of Cofactor Enzyme Activation:

Binding of one oxygen to hemoglobin activates other sites to enhance oxygen binding at the

three other sites.

3a. Enzyme Concentration• What does the

graph tell us about the effect of increasing [enzyme]?

Rate

of R

eacti

on

Enzyme Concentration

• Increasing the [enzyme] means the rate of reaction keeps increasing.

3b. Substrate Concentration

• Increasing [substrate] only increases the reaction rate until all the enzyme active sites are in use.

• What does the graph show us about [substrate]?

4. Enzyme Inhibitors• Competitive inhibitors bind to the active site of

an enzyme, competing with the substrate.

• Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective.

• Noncompetitive inhibition is also called allosteric regulation.

LE 8-19Substrate

Active site

Enzyme

Competitiveinhibitor

Normal binding

Competitive inhibition

Noncompetitive inhibitor

Noncompetitive inhibition

A substrate can normally bind to the

active site of an enzyme.

A competitive Inhibitormimics the substrate,

competing for the active site.

A noncompetitive inhibitor bindsto the enzyme away from theactive site, altering the shape

of the enzyme so that itsactive site no longer functions.

AZT and AIDS• AZT is the first retroviral

drug approved to fight the HIV virus.

• It is a competitive inhibitor for the enzyme reverse transcriptase.

Allosteric Regulation of Enzymes• Allosteric regulation is the term used to describe cases

where an enzyme’s function is affected by the binding of a regulatory molecule at a different site than the active site.

• Allosteric regulation may either inhibit or stimulate an enzyme’s activity.

• Usually the end product of a long series of reactions that make a metabolic pathway inhibits an enzyme near the beginning of the pathway.

• This way when the [product] is high the chemical reactions leading to the product are slowed down.

Allosteric Regulation

Homework

• Read MHR p 41 – 50• Do p 54 # 1 – 11, 13• Adapt and redraw the diagram on p43 to

illustrate an anabolic reaction instead of a catabolic reaction.

• Note: anabolic = synthesiscatabolic = hydrolysis or

breakdown

Feedback Inhibition

Active siteavailable

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Enzyme 2

Intermediate A

Isoleucineused up bycell

Feedbackinhibition Active site of

enzyme 1 can’tbindtheoninepathway off

Isoleucinebinds toallostericsite

Enzyme 3

Intermediate B

Enzyme 4

Intermediate C

Enzyme 5

Intermediate D

End product(isoleucine)

Substrates

Enzyme

Products

Substrates enter active site; enzymechanges shape so its active siteHolds the substrates (induced fit).

Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.

Active site acts on substrates

Substrates areconverted intoproducts.

Products arereleased.

Activesite is

availablefor two new

substratemolecules.

Quick Review ofInduced Fit Model of Enzyme Action

Enzyme-substratecomplex