Proteins are the most common large molecules in living things. They make up about 50 per cent of all organic matter. Proteins are important structural parts of the tissues of cells and of the substances between the cells. They also have an essential role in the control of chemical reactions in the body. Enzymes (discussed in the following article) and many hormones (discussed later in this section) are proteins.
Amino acids
Just as carbohydrates and lipids are made of simpler molecules, all proteins are constructed from amino acids. There are around 20 common amino acids. These form a kind of alphabet from which all the words and sentences in an almost infinitely large book can be made. There is another group of amino acids, many of which are simple derivatives of the common types. These are occasionally
Amino acid
Condensation
Dipeptide
Hydrolysis
Polypeptide
1 1 H—N—C—CO-
H H I I -N— C---- CO-
&
Hemoglobincontains four heme groups, one of which is shown below. Each heme has an iron atom at its center, linked to two nitrogen atoms (pale blue). Oxygen bonds reversibly to the iron, is transported through the bloodstream, and taken up by various cells. This explains the vital role of hemoglobin in cell metabolism.
Polypeptides and proteinsare formed from amino acids by a condensation reaction (red arrows). This reaction forms a dipep-tide connected by a peptide bond. Repetition of this reaction (polymerization) converts dipeptides to polypeptides and these in turn to proteins. A standard formula for an amino acid, with the variable group R, has been used in the diagram on the right. Breakdown of proteins to polypeptides to amino acids is the reverse process, called hydrolysis (blue arrows).
found as part of a specific protein in a particular species. With the exception of two of them, all the amino acids contain just four elements: carbon, hydrogen, oxygen, and nitrogen. The two exceptions are methionine and cysteine, each of which contains a single sulfur atom.
It is the grouping of the four elements in amino acids that makes them so important. Small groupings of amino acids are known as peptides. If the amino acids join together with the loss of a molecule of water, they form dipeptides. The process can be repeated several times until a polypeptide is formed. Longer chains, with perhaps more than 100 units, are referred to as proteins.
In every protein amino acid, a single carbon atom carries four important bonds. Attached to it are two unlike compounds, a hydrogen atom, and a side chain, the nature of which defines the particular amino acid. Thus, the carbon atom at the center of an amino acid has four different groups attached to it. It is therefore asymmetric. Glycine is the only exception. Asymmetric means that one half of the carbon molecule is not a mirror image of the other half. The amino acid is not balanced (symmetrical).
Hemoglobincontains four heme groups, one of which is shown below. Each heme has an iron atom at its center, linked to two nitrogen atoms (pale blue). Oxygen bonds reversibly to the iron, is transported through the bloodstream, and taken up by various cells. This explains the vital role of hemoglobin in cell metabolism.