Enzymes

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Enzymes are Biological Catalysts. They increase the rate of Metabolic reactions. Almost all Biological Reactions involve Enzymes. All enzymes are Globular Proteins with a specific Tertiary Shape. They are usually specific to only one reaction.

The part of the Enzyme that acts a Catalyst is called the Active Site. The rest of the Enzyme is much larger and is involved in maintaining the specific shape of of the Enzyme.

When a reaction involving an Enzyme occurs, a Substrate is turned into a Product. The Substrate can be one or more molecules. The Active Site of an Enzyme is Complementary to the Substrate it catalyses.

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Lock and Key

The Lock-and-key Hypothesis is a model of how Enzymes catalyse Substrate reactions. It states that the shape of the Active Sites of Enzymes are exactly Complementary to the shape of the Substrate.

When a substrate molecule collides with an enzyme whose Active Site shape is complementary, the substrate will fit into the Active Site and an Enzyme-Substrate Complex will form.

The enzyme will catalyse the reaction, and the products, together with the enzyme, will form an Enzyme-Product Complex. According to this model, it is possible for an enzyme to catalyse a reverse reaction.

The Induced-Fit Hypothesis states that the shape of Active Sites are not exactly Complementary, but change shape in the presence of a specific substrate to become Complementary.

When a substrate molecule collides with an enzyme, if its composition is specifically correct, the shape of the enzyme's Active Site will change so that the substrate fits into it and an Enzyme-Substrate Complex can form. The reaction is then catalysed and an Enzyme-Product Complexforms.

Most reactions in a cell require very high temperatures to get going, which would destroy the cell. Enzymes work by lowering the Activation Energy of a reaction.

The Activation Energy of a reaction is lowered by putting stress on the bonds within a molecule, or by holding molecules close together. This increases the likelihood of a reaction, and so lowers the energy required to begin it.

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Factors affecting enzyme activity:

Temperature: enzymes work best at an optimum temperature.

An increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate will too.

Above the optimum temperature, and the enzymes are denatured. Bonds holding the structure together will be broken and the active site loses its shape and will no longer work.

pH:

As with temperature, enzymes have an optimum pH. If the pH changes much from the optimum, the chemical nature of the amino acids can change.

This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be denatured.

Enzyme concentration:

At low enzyme concentration there is great competition for the active sites and the rate of reaction is low. As the enzyme concentration increases, there are more active sites and the reaction can proceed at a faster rate.

Eventually, increasing the enzyme concentration beyond a certain point has no effect because the substrate concentration becomes the limiting factor.

Substrate concentration:

At a low substrate concentration there are many active sites that are not occupied. This means that the reaction rate is low.

When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases.

Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed.

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Cofactors

Most enzymes require additional help from cofactors, of which there are 2 main types:

Coenzymes - these are organic compounds, often containing a vitamin molecule as part of their structure.

Coenzymes are not permanently bound to the enzyme but may be temporarily and loosely bound for the duration of the reaction and then move away once it is completed.

Metal ions - most speed up the formation of the enzyme-substrate complex by altering the charge in the active site e.g. amylase requires chloride ions, catalase requires iron.

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Inhibitors

Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable.

Reversible inhibitors:

Competitive reversible inhibitors: these molecules have a similar structure to the actual substrate and so will bind temporarily with the active site. The rate of reaction will be closer to the maximum when there is more 'real' substrate, (e.g. arabinose competes with glucose for the active sites on glucose oxidase enzyme).

Non-competitive reversible inhibitors: these molecules are not necessarily anything like the substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the enzyme though, so the reaction rate decreases.