Proteins Perform Multiple Functions: Enzymes, Receptors, Ion Channel Proteins

As emphasized in the preceding chapter, there are an unimaginably large number of possible protein structures based on the sequence of amino acids along the amino acid chain. Through the process of evolution, nature has chosen a minute fraction of them to create proteins that provide for the necessities of life. Among all the functions that proteins serve in living organisms, I focus on the three that relate most directly to my tales of drug discovery: catalysis, information transfer, and control of the intracellular milieu. These functions are served by, respectively, enzymes, receptors, and ion channel proteins. I spend most of the time discussing enzymes, because most of the stories in the later chapters focus on enzymes. There are occasions when I refer to receptors and ion channels, as well, but because enzymes are the stars of the stories, let’s start there. Chemical reactions are processes during which one or more molecules are converted into different ones. Chemical reactions involve breaking and forming of the chemical bonds that hold atoms together in molecules. Certain chemical bonds in the starting molecules (the reactants) are broken, followed by the formation of new ones, leading to the end products. All the atoms in the reactants are found in the products; they are just rearranged. A simple example is provided by diamond and graphite. Diamond is brilliant and the hardest natural substance known; graphite is black and very soft. Yet, both diamond and graphite are composed entirely of carbon atoms. The carbon atoms are linked differently by the chemical bonds holding them together, yielding substances with very different properties. It may surprise you to know that graphite is actually very slightly more stable than diamond. So if we wait long enough, the chemical reaction . . . Diamond → Graphite . . . might be expected to occur. However, do not search for evidence of black dots in your wedding diamond. This may be the slowest chemical reaction of all and may take longer than the age of the universe to get anywhere.