Revision:Enzymes 1

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Overview

• Enzymes are biological catalysts

• They consist of one or more polypeptide chains

• They operate by providing an alternative pathway for a reaction to occur, one with a lower activation energy.

• This requires the substrate(s) to bind as ligands to the active site (the area where the reaction occurs) and form an “enzyme-substrate complex”

The “Lock and Key” Hypothesis

• Proposed in 1894 by Fischer

• He noticed that enzymes were able to distinguish between very similar substrates, he reasoned therefore that the enzyme must be complimentary to the substrate like a key is complimentary to a lock.

The “Induced Fit” Hypothesis

• Proposed by Koshland in 1960

• He discovered that when the enzyme substrate complex was formed the enzyme underwent a change in conformation.

• This meant that the complementation between the enzyme and substrate was not exact but the change in conformation lead to a perfect fit.

Inhibition

Enzyme inhibitors are molecules that bind to enzymes and decrease their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors.

Competitive Inhibition

• The inhibitor is capable of binding with the active site of the enzyme preventing the substrate from binding to the enzyme.

• This can either be temporary or permanent the latter of which effectively destroys the enzyme.

• An alternative kind of competitive inhibition is where the inhibitor binds to a separate site, however (and this is important,) the presence of the substrate in the Active site must lead to a conformational shift closing the site of inhibition. If the inhibitor and substrate do not compete then it isn’t competitive inhibition.

Uncompetitive Inhibition

• The inhibitor binds only with the enzyme substrate complex.

• This leads to a reduction in the effective concentration of the E-S complex which then causes the affinity of the enzyme for the substrate to increase, (Le Châtelier's principle.)
• This then causes a reduction in the efficiency of the enzyme as the substrate and enzyme take longer to separate.

Non-competitive inhibition

• The inhibitor binds at an allosteric site (distant from the active site) causing a change in the conformation of the enzyme stopping the substrate binding.
• The presence of the substrate in the active site has no effect on the shape of the site where the inhibitor binds. (There is no competition, hence the name.)

End-product inhibition

Cells need certain substances to function properly. However, an excess of these substances can be poisonous and possibly fatal. This is controlled by a special mechanism called end-product inhibition.

All metabolic pathways require different enzymes for their different stages. In end-product inhbition, the final product of the reaction acts as an inhibitor on one of the enzymes required in the earlier stages of the pathway. This prevents the reaction cycle from continuing, and thus keeps the required substance at a limited concentration.

In the diagram below, A, B, C and D are products and A1, B1 and C1 are enzymes.

A------(A1)-----> B -------(B1)-------> C -----------(C1)-------> D

D may inhibit A1, for example, meaning that once D is produced the reaction ceases.

When more of the substance is required, a special molecule known as an activator may re-activate the necessary enzyme(s).

Other Effects on Enzyme Activity

Temperature

• As the temperature increases, the kinetic energy of the substrate and enzyme molecules increases and so they move faster. The faster these molecules move, the more often they collide with one another and the greater the rate of reaction.
• At high temperatures this vibration increases to such an extent that the interactions between the protein chains are weakened and broken. This causes the three dimensional shape of the enzyme to change meaning it can no longer perform its catalytic action. It is said to be denatured.

pH

• There are many ionic interactions between amino acid side chains, for example between NH₃⁺ and COO^- groups. These are pH dependant.

• In the presence of a better acid the amino group will accept a proton from that rather than the Carboxylic acid in the side chain.

• In the presence of a better base the Carboxylic acid will donate it’s proton to that base rather than to the amino group in the side chain.

• In either case this will break the peptide bonds holding the enzyme's tertiary structure together. This is denaturation.

• Thus, enzymes function at an optimum pH (graphically, this would be represented as a peak at a small range of pH values with a sharp decline either side). Enzymes may have a different optimum pH - for example, peptidases in the stomach are tolerant to the high acidity of HCl.

Substrate Concentration

• An increased concentration of substrate will lead to an increased rate of reaction up to the point when all the available active sites are full due to the increased chance of a collision occurring.

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