Below is a diagram representing an enzyme, its special shape designed to hold its reacting chemicals. You can see that it is a long chain bent and twisted into the necessary shape. How does it get bent in just the right places so that its unique chemicals fit exactly into this ‘jig?’
|Diagram to show how amino acids with different ‘shapes’ are joined together to produce the unique three-dimensional structure of an enzyme.|
If you placed a row of square bricks end to end they would obviously form a straight line. If you introduced into the row a brick with a triangular cross-section, a bend in the row would be obtained. An enzyme molecule is constructed on this principle, using chemicals called amino acids as its ‘bricks’. There are about 20 different amino acids and in effect, they are all different ‘shapes.’ Also, some amino acids have the property of ‘clipping on’ to others further down the chain, thus creating a loop. By careful selection of the various amino acids (and there are usually many hundreds in the enzyme chain) the molecule can be bent into the requisite three-dimensional shape.
Now the important thing! Obviously, to produce a given enzyme there is only one correct sequence of amino acids. The substitution of just one amino acid in the sequence could produce a ‘bend’ in the wrong place, with the result that the enzyme would be unable to hold its particular chemicals and would thus be useless.
So the cell in some way has to remember the correct sequence of amino acids in every one of the hundreds of different enzymes it needs, so that it can make them when required. If it gets even one amino acid in the wrong place in the line, the enzyme might not work properly. How does the tiny cell ensure this correct sequence?