EnzymesPart 2

The features of metabolic pathways are as follows:

1. They consist of many chemical reactions that

are carried out in a particular sequence.

2.  An enzyme catalyses each reaction.

3.  All reactions occur inside cells.

Metabolic Pathways

Some pathways build up organic compounds (anabolic pathways) and some break them down (catabolic pathways).

Some metabolic pathways consist of chains of reactions.

An example of a chain of reactions would be glycolysis where a chain of ten enzyme-controlled reactions convert glucose into pyruvate.

Some metabolic pathways are cycles of reactions where a substrate is continually regenerated by the cycle.

The Krebs cycle is an example of a cycle of reactions.

This model is an extension of the lock and key model.

When the substrate enters the active site of the enzyme it induces or causes a change in the shape of the active site so that it binds more snugly to the enzyme.

The entry of the substrate causes the induced fit and this helps account for the broad specificity of some enzymes.

This means they have the ability to bind with more than one substrate.

The Induced Fit Model

Activation Energy Reactants are converted to products during

chemical reactions. However, before the reactant can take part in

the reaction it has to gain some energy and this energy is referred to as the activation energy of the reaction.

The activation energy is needed to break bonds within the reactant.

As new bonds are formed later in the reaction, energy is given off.

In exergonic reactions this amount of energy is greater than the activation energy and in endergonic reactions it is less.

Enzymes function to lower or reduce the activation energy of the reactions that they catalyse and therefore make it easier for chemical reactions to occur.

The active site of enzymes contains an environment which causes changes to the substrate.

It weakens the bonds of the substrate which is changed into its transition state.

The transition state that is achieved when the substrate binds to the active site has less energy and this is how enzymes are able to reduce the activation energy of reactions.

an inhibiting molecule structurally and chemically similar to the substrate molecule binds to the active site, preventing the substrate from binding.

The substrate and the inhibitor are competing for the active site.

The activity of the enzyme is inhibited until the inhibitor molecule dissociates from the active site.

An increase in the concentration of the substrate will gradually reduce the effect of the competitive inhibitors.

Competitive Inhibition

An example of competitive inhibition would be the inhibition of butanedioic acid (succinate) dehydrogenase by propanedioic acid (malonate) in the Krebs cycle.

Another example would be folic acid synthesis in bacteria by the sulfonamide Prontosil (an antibiotic).

the substrate and inhibitor are not similar and the non-competitive inhibitor will bind to the enzyme at a different site from the active site.

Once it binds with the enzyme the inhibitor changes the conformational change in the active site of the enzyme.

This can prevent a substrate from binding or the substrate may still be able to bind but the reaction is not catalyzed resulting in a decrease in enzyme activity.

Non-competitive Inhibition

Because the substrate cannot prevent the binding of the inhibitor, even at high substrate concentrations some enzymes will remain inhibited and enzyme activity is reduced.

An example of non-competitive inhibition would be metal ions including copper Cu2+, mercury Hg2+, and silver Ag+ inhibiting many enzymes by binding reversibly to the –SH groups of cysteine, the amino acid that forms disulfide bridges.

Allostery is a form of non-competitive inhibition.

In end-product inhibition, the product of the last reaction in a metabolic pathway inhibits the enzyme that catalyses the first reaction.

The enzyme that is inhibited by the end products is known as an allosteric enzyme.

Allosteric enzymes have two non-overlapping binding sites. One of the sites is the active site and the other is the allosteric site.

End-Product Inhibition

In end-product inhibition, the end product acts as an inhibitor by binding to the allosteric site and altering the structure of the enzyme so that the substrate is less likely to bind to the active site.

This process is reversible so that when the

end-product detaches, the enzyme can return to its normal shape.

This method of controlling metabolic pathways has its advantages.

If there is an excess of end-products the

entire pathway is shut off and intermediates do not build up.

Also, as the level of end-products drops, the enzymes that catalyze the first reaction will start working and the metabolic pathway is activated again.

End-product inhibition is an example of negative feedback.