7.6 Enzymes (AHL)

Pp 210 - 213

Pp 69 -71

Metabolic pathways Metabolic reactions often

occurs in small step reactions with a number of intermediate products in-between

Metabolic reactions includes: Catabolic reactions such

as respiration & Anabolic reactions such

as photosynthesisEach step in a metabolic

pathway each is unique & requires its own enzyme to allow finer control of metabolic pathway

Induced-fit model

lock & Key hypothesis does not explain the broad specificity of some enzymes

the molecular shape of active sites is not always complementary to that of the substrate

induced-fit model attempts to over come these difficulties when substrate binds to the enzyme, the shape of active site

changes to accommodate the substrate bonds in substrate weaken & activation energy decreases the structure of the enzyme allows for a certain amount of

adaptation to the substrate, this explains broad specificity of some enzymes such as proteases

Effect of enzymes on activation energy Enzymes lower the activation

energy of the chemical reaction that they catalyse - Ea

In the activated complex or transition state energy is put into the substrate to weaken the structure. This allow the reaction to occur with a minimal amount of additional energy required.

Normal activation energy would denature the proteins of the cell. Thus reduced activation energy make these reactions possible in a cell.

After the product is formed energy is released – ΔG

Exergonic reactions release more energy than the activation energy

Competitive and non-competitive inhibition

Inhibitors are substances that reduce or completely stop the action of an enzyme

Inhibitor can attach on the active site (competitive inhibitor) or on another region of the enzyme molecule other than the active site(non-competitive inhibitor)

Comparing competitive and non-competitive inhibition of enzymes

both types of inhibitor reduce enzyme activity

both types of inhibitor bind to the enzyme

both types of inhibitor prevent the substrate from binding to the active site

Difference between competitive and non-competitive inhibitionCompetitive inhibition Non-competitive inhibition

example: succinate dehydrogenase is inhibited by malonate

substrate and inhibitor are chemically similar & have same shape

inhibitor binds to active site

inhibitor does not change the shape of the active site

increases in substrate concentration reduce the inhibition

example: pyruvate kinase is inhibited by alanine

substrate and inhibitor are chemically not similar & have different shape

inhibitor binds away from the active site i.e. at allosteric site

inhibitor changes the shape of the active site

increases in substrate concentration do not affect the inhibition

Control of metabolic pathways by end-product inhibition

end-product inhibition is an example of negative feedback

it controls rate of product synthesis i.e. amount of product produced

when there is too much of end-products, the product inhibits the enzyme that catalyses the 1st reaction (allosteric enzyme)

the product binds to the enzyme at an allosteric site i.e. site other than the active site

enzyme changes shape & the substrate cannot bind to the active site

binding of inhibitor to an allosteric site is reversible i.e. when the inhibitor detaches, the active site returns to the original shape & substrates can bind again

end-product inhibition controls the metabolic pathway by switching on & off the initial stage of the metabolic pathway based on the concentration of end-product

e.g. ATP inhibition of phosphofructokinase, in glycolysis

Revision QuestionsDescribe the induced-fit

model of enzymes.Outline how enzymes lower

the activation energy of the chemical reactions that they catalyse.

With reference to one example of each, distinguish between competitive and non-competitive inhibition.

Explain the control of metabolic pathways by end-product inhibition.