Protein

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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 formed by joining the ɑ-carboxyl group of an amino acid to the ɑ-amino group of another amino acid[1]. A protein can be made up of a single polypeptide chain or multiple polypeptides linked together. There are three types of proteins: fibrous, globular and membrane proteins. Examples of proteins include enzymes, receptors and hormones.  They are found in every form of life from viruses to bacteriayeasts to humans. One important technique used to analyse proteins is SDS polyacrylamide-gel electrophoresis (SDS-PAGE).

Contents

Structure

A protein has several 'layers' of structure [2]. The function of the protein is determined by it's structure, therefore each layer is dependant on the next.[3] 

Primary Structure

The primary structure is the basic 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: the gene

Secondary Structure

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. The secondary structure is stabilised by hydrogen bonds between the C=O and H-N groups[4]

Tertiary Structure

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. Disulphide bridges are formed between the amino acid Cysteine [5].

Quaternary Structure

One or more tertiary stuctures of protein linked together 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 an iron-containing haem group. Two of the subunits are alpha, and two are beta [6].

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 forming 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 dyneins. Kinesins facilitate the carrying of organelles toward the positive end of the microtubule and dyneins are important of the movement of cilia or flagella in organisms [7].

See also

References

  1. Berg et al., (2006) Biochemistry, 6th edition, New York. Pg 34
  2. Elliott.W.H, Elliott.D.C (1997) Biochemistry and Molecular Biology. New York, United States:Oxford University Press.pp.47-49.ISBN 0199271992
  3. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
  4. Clark, J (2004) The Structure of Proteins. [Internet], Available from: http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html;[Accessed 20 October 2015].
  5. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
  6. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
  7. Alberts.B et al, (Fifth Edition); Molecular Biology of the Cell; Taylor and Francis Group, pp 1014-1015

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