The basics - continued 2

Enzymes

Enzymes are important proteins because they can substantially speed up chemical reactions and enhance their specificity. They are sometimes referred to as biocatalysts. Catalysts are compounds that influence chemical reactions without being used up as a result. In other words, they are reusable. A single enzyme molecule process about 100.000 to 5 million molecules every minute.

The molecules that are processed are called substrates. They are converted to products.

Enzymes are commonly named after the process the catalyse in a name ending with ase. For instance, amylase cleaves starch (amylose), lipases cleave fats (lipids), proteases cleave proteins, etcetera.

The function of a protein is strongly correlated to its amino acid sequence and its tertiary structure. Enzymes have an active site which shape selects the molecules that can and cannot be bound.

The structure of enzymes can help us explain why certain substances are harmful. The harmful molecules interact with the active site of the enzyme. As a result, the active site becomes occupied and therefore cannot perform its catalytic role. Compounds that block the catalytic process are called inhibitors.

All enzymes are proteins, but many have a non-protein part as well: the co-enzyme. This co-enzyme can be a metal ion (Zn²⁺, Ca²⁺, Mg²⁺), a source of energy (ATP) or simply a proton.

A single different amino acid can make an enzyme much slower or even inactive. Alcohol dehydrogenase (ADH) is an example of this (see the image on the right). The variant common in people of Asian descent is much less effective. Besides ADH another enzyme used in processing alcohol, ADLH. This enzyme is also slightly different in many Asian people: amino acid 487 of the sequence is a lysine instead of a glutamate.

Exercise 10:

Draw the amino acids mentioned above and point out the differences. Answer

Exercise 11:

Your red blood cells contain haemoglobin, a protein that transports oxygen trough the body. Carbon monoxide (CO) can take the place of oxygen in the active site of haemoglobin. This means that oxygen cannot longer be transported properly, resulting in headache, nausea, loss of conscience, and in the worse case death.

1. Which essential co-enzyme does haemoglobin need to function? Answer
2. What do we call a compound like carbon monoxide in this case? Answer

Enzymes have a very specific function. They catalyse the conversion of a single compound or they speed up a single type of reaction. This means that they should be able to recognise specific molecules.

Near its active site an enzyme has a cavity or cleft that exactly fits its substrate molecule. This is similar to how a key fits into a lock. The lock will not open if you use the wrong key; the enzyme will not work for the wrong substrate.

Besides specificity for a certain substrate, enzymes also have a pH-optimum and a  temperature optimum. These are the conditions in which the enzyme can catalyse reactions the fastest. Often, an enzyme is only effective over a small temperature or pH range, for instance between 30 and 40°C.

Exercise 12:

1. What is the optimal  is de optimal temperature for enzymes that digest food in human intestines, for instance trypsin?   Answer
2. What is the optimal pH for the enzymes that work in our stomach? Answer

Exercise 13:

Many laundry detergents contain enzymes that break down the stains in your clothing. Proteases, for instance, break down other proteins, lipases break down fats, etc. Could you use human proteases to do your laundry? Answer

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