Primary Structure of Proteins

The primary structure of a protein is simply the specific, precisely ordered sequence in which its amino acids are joined together, one after another, by peptide bonds along the polypeptide chain. By convention, this sequence is always read starting from the end of the chain that has a free (unreacted) amino group -- called the N-terminus -- through to the opposite end, which has a free carboxyl group -- called the C-terminus.

Primary structure is considered the most fundamental level of a protein's structure because it is the only level that is directly specified by the genetic code, and because every higher level of structure that a protein adopts is built entirely on top of it. Once a polypeptide chain's exact amino-acid sequence exists, that sequence -- through the particular pattern of hydrogen bonding, attraction, and repulsion it creates between amino acid side chains and backbone atoms along the chain -- determines how the chain then folds and coils into its higher-order shapes (WBCHSE's syllabus for this chapter covers primary structure only, so these higher levels are mentioned here only to show why primary structure matters, not as separate topics to be studied in detail). …