A look at how amino acids go to make up polypeptides and how polypeptides go to make up proteins.
Proteins are key components of life itself; chemically and physically diverse they perform a wide variety of functions. Protein diversity has meant that there are a number of different ways to classify these molecules. Proteins can be classified by their biological function. Proteins can be either structural, such as collagen, which is found in cartilage, bone and skin or they can be dynamic, actively involved in the processes of life, for example enzymes and hormones are both made of proteins. Proteins can also be classified by their shape, either being fibrous, such as the protein myosin, found in muscles or the protein could be globular, for example in enzymes.
Amino acids
The basic unit of protein is the amino acid. Amino acids are composed of a central carbon atom, covalently bonded with four groups: a hydrogen atom, an amino group, a carboxyl group and an R group. It is this R group that distinguishes one amino acid from another. There are about twenty different amino acids.
Polypeptides
Amino acids are bonded together by peptide bonds to form a polypeptide chain. The peptide bond is formed by a condensation reaction (where the equivalent of a molecule of water is removed from the two joining amino acids). (Finding a diagram of peptide bond formation will help aid understanding of this matter, typing 'peptide bond' into a search engine capable of searching images should find a suitable diagram).
The number of amino acids within a protein can vary greatly; proteins can be small for example adrenocorticotrophic hormone with just 39 amino acids compared to the large invertebrate blood pigment hemocyanin, which has 8200 amino acids.
Structure
It is the order of the amino acids within the polypeptide chain, which makes up the primary structure of the protein. The secondary structure of the protein is the shape of the polypeptide chain; the chain normally forms an alpha helix or a beta sheet in its secondary structure. The secondary structure is held together by ionic bonds, hydrogen bonds and hydrophobic interactions between the amino acids in the chain and the water molecules surrounding it. The tertiary structure of the protein is how the polypeptide chain is folded, this folding is held together by ionic bonds, hydrogen bonds, hydrophobic interactions and disulphide bridges. The quaternary structure of the protein is when it consists of two or more polypeptide chains; the quaternary structure describes how the chains making up the protein are folded around each other.
Sources:
Cell and molecular biology by P. Sheeler and D. E. Bianchi, 3rd edition, Wiley
Protein structure and function
