Enzymes are protein molecules that act as biological catalysts through their ability to increase the rate of the reaction without being used up. Enzymes are fundamental to cellular metabolism because without them reactions in the body would be too slow to enable functioning.

How they work?

Enzymes work by binding a substrate into a cavity known as an active site forming an enzyme-substrate complex and then releasing the product, the enzyme is then free to catalyse further reactions. The binding of the enzyme and the substrate works like a lock and key mechanism and hence is called the lock and key model. The reality is however, the enzymes do not always have a perfect it so we often refer to it being a induced fit model.

Activation energy is the minimum energy required for a reaction to occur, through a substrate binding to enzyme there is a reduction in the activation energy so the energy barrier that is required for the reaction to proceed is lower, so it can occur more easily. Enzymes do not change the amount of product produced they only have affects on the rate of a reaction.


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Factors that affect enzyme activity


  • Enzymes have optimal temperatures to operate at maximal capacity, if this temperature is exceeded the enzyme may denature (change in the enzyme’s active site rendering it inactive).
  • Lower temperatures than optimal do not denature the protein, but the activity of the enzyme will be reduced.


  • Enzymes operate at an optimal pH. Enzymes that are exposed to pH out of their optimal range may denature.

Substrate concentration

  • Increasing the substrate concentration will increase the rate of the reaction until all the enzymes are saturated with substrates, at this point enzyme activity declines.

Enzyme concentration

  • Increasing the enzyme concentration will increase the rate of reaction up until all the substrate has ran out.


Enzyme inhibition

Competitive inhibition

A molecule competes with the substrate for binding to the enzyme’s active site, preventing the binding of enzyme and substrate.

Non competitive

A molecule that binds to an enzyme at another part other than the active site which causes a conformational change to the enzyme’s active site so the substrate can no longer bind.

Rational drug design

In recent times  drugs have been synthesised to block active sites of particular enzymes in order to combat some decisions. For example, Relenza blocks the action of an enzyme known as neuraminidase, which is needed to release viruses from infected cells.

Coenzymes/ cofactors

Some enzymes require another chemical component to attach to the protein in order to work. If organic we refer to them as coenzymes, if inorganic we refer to them as cofactors.

See also

  1. http://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme