The chemical composition of proteins in biological cells

Proteins constitute 80-85% of the dry weight of cells. This reflects their crucial role in life. A protein is composed of chains or units of varying numbers. Their organization and the way they are arranged vary depending on the type of protein. Each unit consists of polypeptide chains. Each polypeptide chain is composed of building blocks known as amino acids. An amino acid consists of a central carbon atom, known as an alpha carbon atom, to which a carboxyl group is attached on one side and an amino group on the other. It also has a side chain known as the "R" group (Figure 1). Amino acids differ from one another based on the presence of the "R" group. In the amino acid glycine, the "R" group may represent hydrogen. This group may be a branched carbon chain, as in leucine, or it may be an unbranched carbon chain, as in glutamine. It may also include an aromatic ring, as in tyrosine (Figure 2). The chemical structure of amino acids gives proteins a great ability to bond with many compounds in the cell. The R group may be nonpolar and hydrophobic, which helps it dissolve in fats, as in the amino acids leucine, isoleucine, and valine. Alternatively, the R group may be polar, dissolving in water by forming hydrogen bonds, as in serine, tryptophan, and histidine (Figure 3).

The R group may be ionized with a negative charge, as in aspartic acid and glutamic acid, or ionized with a positive charge, as in histidine, arginine, and lysine. Or it may be non-ionized, as in cysteine, threonine, serine, glutamine, and asparagine (Figure 4). Furthermore, the R groups in amino acids have varying bonding capacities due to their wide diversity. 

The presence of both amino and carboxyl groups in amino acids gives them an amphoteric nature. An amino acid can be positively or negatively charged, depending on the pH of the solution. It is also neutral in neutral solutions. In proteins, the carboxyl group of one amino acid is linked to the amino group of the following amino acid, forming a polypeptide chain according to the sequence of genetic codes encoded on messenger RNA (Figure 5-6). These polypeptide chains are linked together in an organized manner to form a protein. 

This protein structure gives the protein a number of active sites, enabling it to interact with other organic compounds. Proteins may be created in a simple, free form or linked to other compounds within the cell, as in lipoproteins and carbohydrates (Figure 7). Proteins can also bind to inorganic compounds or elements, as in the binding of globin to iron in red blood cells, forming hemoglobin, and the binding of globin to magnesium in plant chlorophyll (Figure 8).