Why are enzymes so specific

Specificity of the enzymes

Enzymes are substrate specific

Enzymes are globular proteins with a very specific structure: They have what is known as an active center and usually also - at another point on the protein - an allosteric center.

Active center

The active center is the reaction center of the enzyme. Here the substrate fits into the lock like a key, highly specific and complementary.

Here you can see the active center of a (hypothetical) enzyme that reacts formic acid (left) and methanol (right) to form methyl formate. Formic acid and methanol are the two substrates, and the active site exactly matches the structure of the two molecules. Only these two molecules can settle in the active center. Another acid or another alcohol is too big for the active center, it doesn't fit.

The complex of substrate (s) and enzyme is called the enzyme-substrate complex.

In order for the two substrate molecules to react, the active center must somehow change; it has to let the two molecules react with each other. The universal energy carrier ATP is mostly used for this chemical work. By splitting into ADP and inorganic phosphate, energy is released, which then does the chemical work. The enzyme-product complex is formed from the enzyme-substrate complex.

Then the active center of the enzyme has to change a little again so that the reaction product is released and can leave the enzyme. After all, the enzyme does not just catalyze a single reaction step, but many thousands to hundreds of thousands of these steps.

Substrate specificity and group specificity

Most enzymes, however, are not 100% substrate-specific. Chemical compounds that look very similar also fit into the active center. However, the enzyme activity, i.e. the speed at which the enzyme works, is then not that high.

Here we see our example enzyme with two other substrates that also fit very well into the active center - two ethanol molecules. Most enzymes have a very small active center that does not fit all of the substrate. Only a small part of the substrate is in the active center. The rest of the substrate that is not in the active site can easily have a different structure without affecting the enzymatic activity. Our example enzyme should be able to process the two ethanol molecules just as quickly and well as the formic acid and methanol molecules. One speaks here of a so-called group specificity. Most enzymes are even group-specific, but there are also a few enzymes that are strictly substrate-specific, i.e. can actually only process one specific substrate.

Stereospecificity

There are molecules that behave like an image and a mirror image. The following picture shows such a molecule:

No matter how skilfully you turn and turn, you won't be able to bring the two molecules together. In chemistry, this type of isomerism is called mirror image isomerism or enantiomerism (see there).

The chemical and physical properties of such enantiomers are completely identical, but enzymes can easily distinguish the two forms from one another. Just as you can't get your left hand into a right glove, the mirror image of a substrate cannot place itself in the active center of an enzyme. So enzymes are stereospecific.

Enzymes are action-specific

The above (hypothetical) enzyme can only convert formic acid and methanol, it is substrate-specific. But maybe there are several ways in which formic acid can react with methanol. However, the enzyme only catalyzes a single one of these possibilities: It allows methanoic acid and methanol to react to methanoic acid methyl ester, not to other possible end products. This is why it is said that enzymes are also effect-specific. Each enzyme can only catalyze one specific chemical reaction.

However, this statement is not entirely true. An enzyme can namely catalyze two chemical reactions: the forward reaction and the reverse reaction. If a molecule of methanoic acid methyl ester were to be placed in the active center of our example enzyme, then methanoic acid and methanol could also be produced again. Enzymes catalyze the entire reaction, not just the forward reaction, but also the reverse reaction.

If some enzymes give the impression that they are only catalyzing the forward reaction, this is because the principle of the smallest force is used here. If the end product of the enzymatic reaction is immediately removed from the reaction mixture, for example by a second enzyme, the chemical equilibrium of the reaction shifts to the right side, to the product side. The likelihood of a reverse reaction decreases sharply when the products are removed from the mixture of substances.