Proteins are linear polymers of amino acids. Genes encode all proteins, and proteins perform essential roles in all genetic processes, including the synthesis of DNA, RNA, and proteins. Some proteins bind to DNA and RNA, and some proteins are enzymes that act on nucleic acids. Some proteins bind to specific nucleotide sequences, but others bind equally well to any nucleotide sequence. This chapter describes the main points of protein size and structure.
Protein is a generic term for a linear polymer made of amino acids as well as an aggregate of these polymers. An amino acid is a small carboxylic acid with an amino group and a side group that defines it. There are hundreds of different amino acids, but only 22 that are known to be genetically encoded, and two of these – selenocysteine and pyrrolysine – are found in only a handful of proteins. The molecular masses of the 20 common, genetically encoded amino acids range between 75 and 204 Da (Figure 3.1). Notice that amino acids are smaller than nucleotides.
A peptide is a short polymer of amino acids, usually 30 amino acids or fewer. Adjacent amino acids in peptides are held together by a peptide bond (Figure 3.2) – a covalent bond between the carboxyl carbon atom of one amino acid and the core amino nitrogen atom of the other. A polypeptide is a large polymer of amino acids, usually 100 amino acids or more.