Enduring Understanding: Competition and cooperation

are important aspects of biological systems

Essential Knowledge 4.B.1: Interactions between molecules affect their structure and function

Life requires enzymes to speed up reactions

Biological Catalysts Reduce Energy Barrier

Enzymes belong to what class of macrmolecule?◦ proteins

How do enzymes work?◦ They lower the

activation energy required to get a reactions started

Life requires enzymes to speed up reactions

The structure of an enzyme determines its function.

Induced Fit of substrate + enzyme = enzyme substrate complex

Why is the shape of an enzyme important?◦ Enzymes work with specific

substrates◦ Enzymes must be able to

bind to the substrate ◦ The substrate must be

complementary to the surface properties (shape and charge) of the active site

◦ The substrate must fit into the enzyme’s active site

Life requires enzymes to speed up reactions

Figure 8.15-3

Substrates

Substrates enter active site.

Enzyme-substratecomplex

Enzyme

Products

Substrates are heldin active site by weakinteractions.

Active site canlower EA and speedup a reaction.

Activesite is

availablefor two new

substratemolecules.

Products arereleased.

Substrates areconverted toproducts.

12

3

45

6

Enzyme Facts:◦ Enzymes are reusable◦ Enzymes are substrate specific◦ Enzymes lower the activation energy of a reaction◦ Enzymes change shape upon binding to the

substrate – thus holding the substrate in such a way to favor breaking existing bonds and making new ones – Induced Fit

Life requires enzymes to speed up reactions

Many enzymes require nonprotein helpers in order to function◦ Cofactors and coenzymes interact with enzymes

to cause structural changes that alter the activity rate of the enzyme

◦ The enzyme may only become active when all the appropriate cofactors or coenzymes are present and bind to the appropriate sites on the enzyme

Interactions between molecules affect their structure and function

What is the difference between a cofactor and a coenzyme?◦ Cofactor – inorganic, such as the metal atoms

zinc, iron and copper in ionic form

◦ Coenzyme – organic molecule; many are vitamins

Interactions between molecules affect their structure and function

Other molecules and the environment in which the enzyme acts can enhance or inhibit enzyme activity. ◦ Environmental factors that may alter enzyme

shape (particularly the tertiary structure) thereby altering enzyme function Temperature pH Salt concentration

Interactions between molecules affect their structure and function

Optimal temperature fortypical human enzyme (37°C)

Optimal temperature forenzyme of thermophilic

(heat-tolerant)bacteria (77°C)

Temperature (°C)(a) Optimal temperature for two enzymes

Rat

e of

reac

tion

Rat

e of

reac

tion

120100806040200

0 1 2 3 4 5 6 7 8 9 10pH

(b) Optimal pH for two enzymes

Optimal pH for pepsin(stomachenzyme)

Optimal pH for trypsin(intestinal

enzyme)

Molecules can bind reversibly or irreversibly to an enzyme’s active site or an allosteric site – changing the activity of the enzyme

Regulation of enzyme activity helps control metabolism Allosteric regulation – (allo refers to “other”) a

regulatory molecule binds to the enzyme at a site other than the active site - can either speed up (activator) or slow down (inhibitor) the reaction

Cooperativity – a substrate molecule binding to one active site in a multi-subunit enzyme triggers a shape change in all the subunits – increasing the reaction

Interactions between molecules affect their structure and function

(a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition

Substrate

Activesite

Enzyme

Competitiveinhibitor

Noncompetitiveinhibitor

Regulatorysite (oneof four)

(a) Allosteric activators and inhibitors

Allosteric enzymewith four subunits

Active site(one of four)

Active formActivator

Stabilized active form

Oscillation

Non-functionalactive site

Inactive form InhibitorStabilized inactiveform

Inactive form

Substrate

Stabilized activeform

(b) Cooperativity: another type of allosteric activation

Feedback inhibition is a common mode of metabolic control. An end product of a metabolic pathway can bind to an enzyme at the start of the pathway – changing the shape of the enzyme thereby “inhibiting” the reaction from taking place

Interactions between molecules affect their structure and function

Active siteavailable

Isoleucineused up bycell

Feedbackinhibition

Active site ofenzyme 1 isno longer ableto catalyze theconversionof threonine tointermediate A;pathway isswitched off. Isoleucine

binds toallostericsite.

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product(isoleucine)

The change in function of an enzyme can be interpreted from data regarding the concentrations of product or substrate as a function of time.

These representations demonstrate the relationship between an enzyme’s activity, the disappearance of substrate, and/or presence of a competitive inhibitor.

Interactions between molecules affect their structure and function

What could we measure?◦ Disappearance of H202

◦ Production of H2O and/or production of O2◦ Heat given off

Catalase + 2H2O2 Catalase/H2O2 complex Catalase + 2H2O + O2