Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

This unit is an explanatory page, referenced by several units dealing with the action of enzymes. As such it has a slightly different format than most units on this website - for example it does not lead directly on to the "next topic" in a sequence. Following popular demand, its format has been converted to give the mouseover gap-fill technique as used on other pages of this site.
Clicking on the yellow screen icons in the right margin can bring in a number of animations which come up as separate pages, each of which can be closed afterwards!
There are now also links - in the bar above - taking you through the series.


How enzymes work

Essential points about enzymes :
- all enzymes are proteins
- each enzyme has a specific catalytic action
- their normal activity depends on their environment
- abnormal conditions cause reduced activity

1 Enzymes are globular proteins - their molecules are round in shape. They have an area - usually thought of as a pocket-shaped gap in the molecule - which is called the active site.
Some enzymes are found inside cells (intracellular enzymes), and some - especially digestive enzymes - are released so they have their effects outside the cell (extracellular enzymes).

The accompanying diagrams are intended to illustrate a generalised account of the action of digestive enzymes. However enzymes are 3-dimensional in shape, unlike the following 2-dimensional graphics.
Enzyme showing active site
2 (Only) the substrate (or substrates) fits/fit into the active site.

In fact the action of enzymes is often explained by comparing it with a lock and key, although a more advanced theory - 'induced fit' - covers the idea that enzymes have a flexible and variable 3-D structure, more like a hand in a glove.

There are several types of enzyme which contribute to different types of biochemical reaction - see below. It is not widely appreciated that water is also a reactant in the digestion (enzyme-controlled breakdown) of most biological molecules. This sort of reaction is known as hydrolysis - meaning 'splitting by water'.
Enzyme and substrate
The enzyme speeds up the process of conversion of substrates (reactants) into products - usually so much that the reaction does not take place in the absence of enzyme.
Although the enzyme obviously joins with the substrate for a short while, the enzyme and substrate split apart afterwards, releasing the enzyme. Thus the enzyme is not used up in the process (unlike the substrate(s)), so it can continue to react if more substrate is provided.

Enzyme and products
animation 1

3 Within the normal range, changes in temperature, pH, and concentrations of substrate and enzyme affect the rate of reaction in accordance with predictable interactions between enzyme and substrate molecules.
animation 2
- The effects of temperature may be explained on the basis of kinetic theory - increased temperature increases the speed of molecular movement and thus the chances of molecular collisions, so within a narrow range (often 0-45 °C), the rate of reaction is proportional to the temperature. It is often said that an enzyme's rate of reaction doubles for every 10 °C rise in temperature. In this respect, the biochemistry of enzymes is similar to chemistry as studied elsewhere, The interaction between this positive effect of increased temperature and the negative effect described below results in a different situation, so that enzymes may be said to have an optimum temperature for their action. However it would be wrong to assume that all enzyme-controlled rreactions in the body have an optimum temperature of exactly 37 ° C.

- Changes in the pH probably affect the attraction between the substrate and enzyme, and thus the efficiency of the conversion process. Often, there is an optimum pH - near to pH 7 (neutral) in intracellular enzymes, and either in the acidic range (perhaps pH 1- 6) or in the alkaline range (pH 8-14) for different digestive enzymes.

- Some enzymes work better if other substances are also present. Some enzymes (pepsin - from the stomach) work better if acid is present (see above), and some, e.g. lipases are more effective if emulsifying agents are present because they break up the substrate into smaller droplets.

animation 3 4 Above normal temperatures (say 60 °C), heat irreversibly alters the enzyme molecule. This denaturation is due to molecular vibrations (caused by heat) which change the shape of the protein, altering the folding and internal cross-linkages in its polypeptide chains. These changes - especially in the region of the active site - mean that the enzyme is inactivated, even when returned to normal temperature.
It would be wrong to say that an enzyme is KILLED by heat, since it is only a molecule, not a living organism.

The higher the temperature to which the enzyme is subjected and the longer the heating is continued, the greater the proportion of damaged enzyme molecules and the result is that the conversion process becomes less and less efficient.animation 4


Below normal temperatures, enzymes become less and less active, due to reductions in speed of molecular movement, but this is reversible, so enzymes work effectively when returned to normal temperature.

Enzymes are sometimes adversely affected by other chemical substances which combine with them, either at their active site or by altering the overall shape of their molecule. These are called competitive and non-competitive inhibitors. Many drugs and poisons have their effect in this way.

Click here for more information about enzymes (examples of enzymes and their action, measuring rate of reaction of enzyme-controlled reactions)

This explanation using stylised graphic representations was developed to assist in the understanding of enzyme action.
Other pages on this site show the 3-dimensional structure of specific enzymes in some detail, and permit different interactive opportunities.


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