What are proteins?
Proteins are biopolymers made of amino acids arranged in a linear chain and joined by peptide bonds. The sequence of amino acids in a protein are specified by a gene. The genetic code specifies 20 "standard" amino acids; some residues in a protein are chemically altered in post-transcriptional modification. Proteins often associate to form stable complexes.
Proteins are essential in all living organisms and participate in every process within cells. Many proteins are enzymes that catalyze biochemical reactions. Other proteins have structural or mechanical functions, such as the proteins in the cytoskeleton, which maintains cell shape. Proteins are also important in signaling, immune responses and the cell cycle. Protein is also a necessary component in our diet, since we cannot synthesise all the amino acids and must obtain essential amino acids from food.
In this chapter, we examine in detail the chemistry and geometry of amino acids, and their basic organisation in proteins. The next chapter will cover protein secondary and tertiary structures.