The amino acids are joined together by peptide bonds in a condensation reaction to form di-peptides
Primary Protein Structure
The amino acids continue to bond through peptide bonding until long chains known as polypeptides have been formed. A polypeptide always has one end with a free amino group and one end with a free carboxyl (acid) group. These polypeptides form the basic, primary structure of a protein, known as the “backbone”.
Secondary Protein Structure
Very shortly after the backbone has formed, it begins to twist, either coiling around in a spiral pattern to form an alpha-helix, or doubling back on itself to form a beta-sheet.
The alpHa-helix forms when the polypeptide chain coils around on itself as aresult of the bonds on either side of the alpha carbon rotating. It is then stabilized by hydrogen bonding between the CO and NH groups.
The Beta-sheet forms when the polypeptide chain twists back on itself to form a sheet like pattern. Hydrogen bonds are again formed to stabilize the secondary structure, however the bonds can occur between CO and NH groups that can be far apart in relation to the length of the chain; they are parallel to one another.
Tertiary Protein Structure
The tertiary structure of a protein is the one that gives a protein its unique shape. The secondary structure of the alpha helix folds back on itself to present a three-dimensional shape. It can also interact with the amino acid R groups. The tertiary structure is stabilized through hydrogen bonds, disulphide bonds and ionic bonds between the R groups of the polypeptide chain. The hydrogen bonds tend to be very weak and are therefore render the protein succeptible to even slight physical changes such as a decrease or increase in pH. Some proteins, for example myoglobin, stop developing at the tertiary stage.
Quaternary Protein Structure
A quaternary protein structure is formed as the result of two or more tertiary structures curling together and is stabilized by weak bonds between the R groups of the different polypeptide chains. Proteins with a quaternary structure can be divided into two main groups: globular proteins and fibrous proteins. Globular proteins such as hemoglobin, insulin and a large number of enzymes are composed of both alpha-helices and beta-sheets and are generally clustered together in a spherical or globular shape. Fibrous proteins are composed solely of either alpha-helices or beta-sheets, for example keratin is composed of many alpha-helices coiled together.
Hemoglobin
