2.4 Amino acids

Amino acids are a group of over a hundred chemicals of which around 20 commonly occur in proteins. They always contain a basic group, the amino group (—NH₂), and an acid group, the carboxyl group (—COOH). Most amino acids have one of each group and are therefore neutral, some have more amino groups than carboxyl ones (basic amino acids) while others have more carboxyl than amino groups (acidic amino acids).

Amino acids are soluble in water. They have both acidic and basic properties, i.e. amino acids are amphoteric. Being amphoteric means that amino acids act as buffer solutions. A buffer solution is one which resists the tendency to alter its pH even when small amounts of acid or alkali are added to it. Such a property is essential in biological systems where any sudden change in pH could adversely affect the performance of enzymes.

2.5 Enzymes

Did you know?

Typically enzymes speed up a chemical

reaction between one million (106) and

one trillion (1012) times – the equivalent

of accelerating a life span of 100 years

Into the space of just 1 second!

Until recently it was thought that all biological catalysts were enzymes. We now know that other substances may carry out catalytic functions in living organisms. Abzymes are antibodies with catalytic properties and ribozymes are molecules of RNA which act catalytically on themselves. Most biological catalysts however are globular proteins known as enzymes. A catalyst is a substance which alters the rate of a chemical reaction without itself undergoing a permanent change. As they are not altered by the reactions they catalyse, enzymes can be used over and over again. They are therefore effective in very small amounts. Enzymes cannot cause reactions to occur; they can only speed up reactions that would otherwise take place extremely slowly. The word 'enzyme' means 'in yeast', and was used because they were first discovered by Eduard Buchner in an extract of yeast.

Enzymes are complex three-dimensional globular proteins, some of which have other associated molecules. While the enzyme molecule is normally larger than the substrate molecule it acts upon, only a small part of the enzyme molecule actually comes into contact with the substrate. This region is called the active site.

Translation

Biology around us. Cholesterol

Cholesterol is a lipid containing four rings of carbon and hydrogen atoms with a branched side chain. A single hydroxyl group (OH^-) gives the molecule a small charge as the result of ionization. Cholesterol is very hydrophobic.

Most cholesterol in the body is found in the membranes of cells. Cholesterol is essential for the functioning of plasma membranes where it plays two main roles. Firstly it limits the uncontrolled leakage of small molecules (water and ions) in and out of the plasma membrane. The cell can thus control the passage of solutes and ions using specialized membrane proteins and without wasting energy counteracts their leakage. The second role of cholesterol in membranes is to pull together the fatty acid chains in phospholipids, restricting their movement, but not making them solid. Cholesterol is also used by the liver for making bile salts and, in small quantities, is used to make steroids.

In total the body contains a pool of about 120-150 g of cholesterol which is maintained by biosynthesis in the liver and intestine and by ingestion of meat, seafood, eggs and dairy products. Vegans take in no cholesterol but most other diets result in an intake of approximately 0.5 g, the body making a further 0.5 g, per day. Cholesterol is lost from the body mainly as bile salts, but also as bile, in cells from the lining of the intestine and as steroid hormones in the urine.

Cholesterol is insoluble in water but can be carried in the blood plasma in the form of lipoproteins. The balance of these lipoproteins is usually maintained by special receptors in the liver cells but saturated fats in the diet decrease their activity and hence lead to a rise in plasma cholesterol. Deposits of crystalline cholesterol and droplets of cholesterol esters can cause thickening of the artery walls (atherosclerosis). This can lead to heart attacks (from blocking of coronary arteries), strokes (brain arteries blocked) or blockages of arteries in the legs. Atherosclerosis may follow damage caused to the artery walls by high blood pressure and smoking. Smoking considerably decreases the concentration of the antioxidant vitamins E and C in the blood resulting in the oxidation of some lipoproteins.

The products of oxidation are often toxic to the cells of the artery and cause them to behave abnormally. The damaged cells release substances which cause the blood to clot and the artery to contract. Macrophages which degrade the oxidized lipoproteins are unable to deal with the cholesterol it carries. Eventually they fill with cholesterol and die, depositing the cholesterol back into the artery.

Speaking

Do you agree with the statement that cholesterol is a fully unnecessary substance in the human body? Speak on its function.

Why is it vital to control the amount of cholesterol in blood?