7.6 Enzymes

Topic 7 Nucleic Acids & Proteins

Metabolic Pathways Metabolic pathways have these features:

They consist of many chemical reactions that are carried out in a particular sequence.

An enzyme catalyses each reaction. All the reactions occur inside cells. Some pathways build up organic compounds (Anabolic). Some pathways break down organic compounds (Catabolic). Some metabolic pathways consist of chains of reactions

eg: glycolysis a chain of 10 enzyme controlled reactions

Some metabolic pathways consist of cycles of reactions: eg: the Krebs Cycle

Metabolic Pathways

Ref: Biology for the IB Diploma, Allott

Induced Fit Model Scientists have discovered the lock and key model of

enzyme action does not fully explain bind of substrates and active sites.

A modification of the lock and key model is the Induced Fit Model.

This model proposes that the active site does not fit the substrate precisely until the substrate binds.

As the substrate binds to the active site, the active site changes shape to better fit the substrate.

This weakens the bonds in the substrate, thus reducing the activation energy required for the reaction.

Induced Fit Model

Ref: Biology for the IB Diploma, Allott

Induced Fit Model Some enzymes have quite a broad specificity:

eg: some proteases

The induced fit model explains this better than the lock and key.

If the shape of the active site changes when the substrate binds, several different, but similar substrates could bind successfully to it and be catalysed.

Activation Energy For reactions to occur, the reactants must meet with

sufficient energy. This energy is called the Activation Energy. Enzymes work by lowering the activation energy. When the enzyme-substrate complex is formed the bonds

of the substrate are stressed/weakened. This means that less energy is required to break them.

Exergonic & Endergonic Reactions Chemical reactions can be either:

Exergonic: This is where energy is released. The products have less energy than the reactants.

Endergonic: This is where energy is taken in from the surroundings. The products have more energy than the reactants.

Ref: Biology for the IB Diploma, Allott

Ref: Biology for the IB Diploma, Allott

Inhibition of Enzymes Some substances reduce the activity of enzymes or even

prevent it completely. These substances are called Enzyme Inhibitors. There are two main classes of inhibitors:

Competitive: Non-competitive:

Competitive Inhibitors Competitive inhibitors are where:

The inhibitor and the substrate are structurally very similar. The inhibitor binds to the active site preventing the substrate

from binding.

An example is the inhibition of folic acid synthesis in bacteria by sulfonamide Prontosil™ (an antibiotic)

Competitive Inhibitors

Ref: Biology Levels of Life, Lecornu

Non-competitive Inhibitors Non-competitive inhibitors are where:

The inhibitor and substrate are not structurally similar. The inhibitor binds to the enzyme at another site, other than the

active site. This changes the shape of the active site and prevents the

substrate from binding. An example is cyanide inhibition of cytochrome oxidase

(an important enzyme in cellular respiration). The cyanide binds to the –SH groups in an enzyme and

breaks the disulphide bridges, changing the shape and function of the enzyme.

Non-competitive Inhibitors

Ref: Biology Levels of Life, Lecornu

Graphical Representation of Inhibition

Ref: IB Biology, OSC

Allostery & Metabolic Pathways A special kind of non-competitive inhibition is Allostery. In many metabolic pathways, the product of the last

reaction inhibits the enzyme that catalyses the first reaction.

This is called end-product inhibition. This is also an example of negative feedback. The site where the end-product inhibitor binds is called

the allosteric site.

Allostery & Metabolic Pathways

Ref: Biology for the IB Diploma, Allott

Allostery & Metabolic Pathways The advantage of this method is that if there is an excess

of the end-product, the whole pathway can be switched off and intermediates do not build up.

If the levels of end-product are low, more and more of the enzymes that catalyse the first reaction will start to work and the whole pathway will become activated.