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6. Proteins as catalysts Page 24
photo of contact lens
Figure 1
Contact lens users have to ensure that they clean and sterilise their lenses regularly. Protein material from dead cells can be particularly difficult to remove. Cleaning solutions contain a proteolytic enzyme to hydrolyse the protein debris into water soluble products that then wash away.
6.1 Enzymes in metabolism
An organism uses the raw materials it has available to supply energy and the chemical building blocks it needs to grow. The chemistry it uses (its metabolism) is complex but we can group the reactions into two simple categories; those that break down large molecules into smaller units (catabolism) and those that build the small units back into different large molecules (anabolism). In general catabolic reactions release energy to a network of small molecules that then make it available for the anabolic reactions to use.

Organisms have enzyme catalysts to make sure that each reaction in its metabolism happens at the correct speed. Enzymes themselves are regulated so that metabolism as a whole is a co-ordinated web of reactions, happening in moderate conditions of temperature, pressure and pH.

Until recently scientists thought all biological catalysts were proteins, but they have discovered that a group of nucleic acid molecules, called ribozymes, act as catalysts in some single celled organisms. In this section, though, we will only look at protein catalysts.

6.2 How enzymes work
Catalysts change the speed of a chemical reaction without suffering any permanent chemical change themselves.

To understand how enzymes work as catalysts we first need a simple picture of what happens to the particles when two chemicals react.

The particles may be atoms, ions or molecules but we will refer just to molecules to keep it simple.

graph of energies
Figure 2
Graph showing the kinetic energies of molecular collisions. Eact is the activation energy for a reaction.
For a reaction to happen the two reacting molecules must:
  • meet
  • collide with at least a certain minimum kinetic energy (the activation energy) so that the necessary bonds are broken (Figure 2)
  • collide the right way round. Molecules have reactive parts (functional groups) and unreactive parts. Collisions that don’t bring together the reactive parts of molecules are unlikely to make a reaction happen whatever the energy.

Any change to the reaction conditions that alters any of these three will alter the speed of a reaction.

To answer these you may need to refer to a standard advanced Chemistry text book.

1. Why are the rates of many reactions increased by increasing the concentrations of the reactants?

2. There are some reactions where the rates do not depend on the concentration of one or more of the reactants. Explain this.

Paw-paws, chewy meat and beer
When an animal is slaughtered its muscle proteins gradually contract and their fibres lock together. This is rigor mortis. A carcass in this state produces very tough meat.

To reduce the toughness, meat producers rely on protein digesting enzymes to hydrolyse the protein fibres partially, so loosening them. Cells in the meat contain enzymes that can do this, but the process is speeded up by adding the enzyme papain.

photo of paw-paws
Paw-paws make the enzyme papain.
Papain is a protein digesting enzyme that comes from the latex of the tropical melon (paw-paw). It is stable and easy to obtain in relatively large quantities. The meat producer may inject a preparation of the enzyme into the animal before or after slaughter. Injection after slaughter allows greater control over how much tenderisation occurs.
Tenderisation using papain makes it possible to reduce the amount of time that the carcass is hung before use and this cuts costs. Papain can also help meat producers remove the maximum amount of meat from each carcass. Even skilled butchers cannot remove more than about 95% of the meat using knives; some is always difficult for them to get to. The enzyme solution reaches this meat and dissolves enough of it to loosen it from the bone.


photo of meat
Papain can help to tenderize meat.
Gelatine is a useful food product made by boiling the bones in water. Papain not only saves wasting meat: it also cleans up the bones so that they produce a high quality gelatine.Brewers use papain in chill-proofing, one of the final stages in beer making. During cold storage beers may go cloudy as proteins precipitate out. Papain breaks down enough of the protein to prevent this happening. The brewers must add the enzyme in controlled amounts, though, as the papain can remain active throughout storage causing too much protein hydrolysis . The a-amino acids produced give the beer too sharp a taste and the reduction in protein prevents the beer from forming a stable head.
Unilever Education Advanced Series: Proteins
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