A protein is a biological polymer which is made up of amino acids. The amino acids are joined together with a peptide bond to form a polypeptide chain. The peptide bond is is formed by joining the ɑ-carboxyl group of an amino acid to the ɑ-amino group of another amino acid. A protein can be made up of a single polypeptide chain or multiple polypeptides linked together. Examples of proteins include enzymes, receptors and hormones. They are found in every form of life from viruses to bacteria, yeasts to humans. One important technique used to analyse proteins in SDS polyacrylamide-gel electrophoresis (SDS-PAGE).
A protein has several 'layers' of structure .
The primary structure is the sequence of amino acids joined togther by peptide bond. There are 20 different amino acids found in nature. This is determined by the DNA sequence that encodes for that particular protein, called the gene.
Secondary structure is the first level of protein folding. The two main folding structures of a protein are the alpha-helix or the beta-sheet depending on the sequence of amino acids. This, in turn, allows the protein to have a hydrophobic core and a hydrophilic surface.
Tertiary structure relates to the protein function. If the tertiary structure is wrong then the protein is unlikely to function properly. Tertiary structure is held together by either hydrogen bonds or disulphide bridges depending on the amio acids present. Finally, if there is more than one peptide chains linked together to form a protein then you get a quarternary structure.
One or more tertiary stuctures of protein build up a quaternary structure. Quaternary structure can also refer to proteins with an inorganic prosthetic group attatched. An example being haemoglobin; a tetramer consisting of four myoglobin subunits and a haem group.
Functions of Proteins
Proteins make up 50% of each cell and have both structural and functional importance. Enzymes are globular proteins that act as biological catalysts and collagen is a fibrous protein which provides strength and structural support in many tissues.
Enzymes work by binding substrate at their active sites, which is a specific region dependant on amino acid sequence, this forms an enzyme-substrate complex. This causes a conformational change in the shape of the enzyme which encourages catalysis by putting strain on the bonds in the substrate (and/or by other means).
A group of protein structures called motor proteins are responsible for activities such as muscle contraction, cell movement, migration of chromosomes during mitosis and the direction of organelles. There are two different types of microtubule motor proteins known as kinesins and dyenins. Kinesins facilitate the carrying of organelles toward the positive end of the microtubule and dyenins are important of the movement of cilia or flagella in organisms.
- ↑ Berg et al., (2006) Biochemistry, 6th edition, New York. Pg 34
- ↑ Elliott.W.H, Elliott.D.C (1997) Biochemistry and Molecular Biology. New York, United States:Oxford University Press.pp.47-49.ISBN 0199271992
- ↑ Alberts.B et al, (Fifth Edition); Molecular Biology of the Cell; Taylor&amp;Francis Group, pp 1014-1015