Enzyme Action

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Enzymes are Biological Catalysts

Enzymes speed up reactions by acting as biological catalysts.

Enzymes catalyse metabolic reactions - both at a cellular level (e.g. respiration) and for the organism as a whole (e.g. digestion in mammals).

Enzymes can affect structures in an organism (e.g. enzymes are involved in the production of collagen, an important protein in connective tissues of animals) and as well as functions (e.g. respiration).

Enzyme action can be intracellular - within cells, or extracellular - outside cells.

Enzymes are proteins.

Enzymes have an active site, which has a specific shape. The active site is the part of the enzyme where the substrate molecules (the substance that the enzymes interact with) binds to.

Enzymes are highly specific due to their tertiary structure (stereospecific due to the chiral nature of amino acids that make up enzymes).

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Enzymes Lower the Activation Energy of a Reaction

In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start. This is called the activation energy - it's often provided as heat. Enzymes lower the amount of activation energy that's needed, often making reactions happen at a lower temperature than the could without an enzyme. This speeds up the rate of reaction.

When a substrate fits into the enzymes active site it forms an enzyme-substrate complex - it's this that lowers the activation energy:

If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily.

If the enzyme is catalysing a break down reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily.

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The Lock and Key Model was a Good Start

Enzymes are specific - they only work with substrates that fit their active site. Early scientists studying the action of enzymes came up with the lock and key model. This is where the substrate fits into the enzyme in the same way a key fits into a lock.

Scientists soon realised that the lock and key model didn't give the full story. The enzyme and substrate do have to fit together in the first place but new evidence showed that the enzyme-substrate complex changed shape slightly to complete the fit. This locks the substrate even more tightly to the enzyme. Scientists modified the old lock and key model and came up with the induced fit model.

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The Induced Fit Model is a Better Theory

The induced fit model helps to explain why enzymes are so specific and only bond to one particular substrate. The substrate doesn't only have to be the right shape to fit the active site, it has to make the active site change shape in the right way as well.

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Enzyme Properties Relate to Their Tertiary Structure

Enzymes are very specific - they usually only catalyse one reaction, e.g. maltase only breaks down maltose, sucrase only breaks down sucrose.

This is because only one complementary substrate will fit into the active site.

The active site's shape is determined by the enzyme's tertiary structure (which is determined by the enzyme's primary structure).

Each different enzyme has a different tertiary structure and so a different shaped active site. If the substrate shape doesn't match the active site, an enzyme-substrate complex won't be formed and the reaction won't be catalysed.

If the tertiary structure of the enzyme is altered in any way, the shape of the active site will change. This means that the substrate won't fit into the active site, an enzyme-substrate complex won't be formed and the enzyme will no longer be able to carry out it's function.

The tertiary structure of an enzyme may be altered by changes in pH or temperature.

The primary structure of a protein is determined by a gene. If a mutation occurs in that gene, it could change the tertiary structure of the enzyme produced.