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A protein is a biological polymer which is made up of structures called 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 bacteria; yeasts to humans. One important technique used to analyse proteins is SDS polyacrylamide-gel electrophoresis (SDS-PAGE). Proteins can make up to 50% of the weight of a cell, and up to 25% of a human's dry bodyweight.



A protein has several 'layers' of structure[2]. The function of the protein is determined by its structure. Therefore each layer is dependent on the next[3].

Primary Structure

The primary structure is the specific sequence of amino acids joined together by peptide bonds in a polypeptide chain. There are 20 different amino acids found in nature. The sequence of amino acids is determined by the DNA sequence that encodes for that particular protein. This is known as the gene.

Primary structure of a protein.

Example of primary structure of a protein[4].

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. Alpha-helix can exists as right-handed or left-handed while beta-sheet can exists as anti-parallel or parallel. 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[5]. for the peptide backbone.


Example of secondary structure of protein[6].

Tertiary Structure

Tertiary structure relates to the protein function. If the tertiary structure is altered, 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 multiple units of the amino acid Cysteine[7].


Example of tertiary structure[8].

Quaternary Structure

One or more tertiary structure of proteins linked together build up a quaternary structure. The quaternary structure can also refer to proteins with an inorganic prosthetic group attached, 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[9].

Functions of Proteins

Proteins make up 50% of each cell and have both structural and functional importance. Proteins transport a multitude of different particles from macromolecules to electrons. Enzymes are globular proteins that act as biological catalysts, and collagen is a fibrous protein which provides strength and structural support in many tissues. Proteins in the form of hormones transmit information between specific cells.

Structural proteins include:

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 a 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[10].

A protein molecule's physical interaction with other molecules determines its biological properties. In some cases, these interactions are very strong; in others, it is weak and short-lived. But the binding always shows great specificity, in the sense that each protein molecule can usually bind just one of a few molecules out of many thousands of different types it encounters[11].

Synthesis of Proteins

Protein synthesis can be divided into two sections, transcription and translation. In transcription DNA is used to code for the protein, it starts at a promotor gene at the 5' end one of the two DNA strands, here RNA polymerase, which does not require primers, moves down the strand and forms a complementary sequences of pre-mRNA. (Thymine base is replaced with Uracil) This pre-mRNA contains non-coding introns and coding exon, due to this, the pre-mRNA is spliced to remove the introns leaving only the coding sequences of mRNA. This mRNA is used to code for the protein sequence.

In translation the mRNA binds to a ribosome, this ribosome then moves down the mRNA from the 5' to 3' end. tRNA has an anticodon sequence with three bases on it that are complementary to a codon on the mRNA, it also carries a specific amino acid. Here the RNA carries this amino acid to the ribosome and its complementary triplet code on the mRNA. Peptide bonds are formed between amino acids next to each other ( when their two triplet codes are next to each other)[12]. This forms the primary structure of proteins which is the amino acid sequence.

See also


  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. Libretexts. Protein Structure. 2017 [cited 09/12/18]; Available from:
  5. Clark, J (2004) The Structure of Proteins. [Internet], Available from:;[Accessed 20 October 2015].
  6. Libretexts. Protein Structure. 2017 [cited 09/12/18]; Available from:
  7. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
  8. Libretexts. Protein Structure. 2017 [cited 09/12/18]; Available from:
  9. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.
  10. Alberts.B et al, (Fifth Edition); Molecular Biology of the Cell; Taylor and Francis Group, pp 1014-1015
  11. Alberts.B et al, (Sixth Edition); Molecular Biology of the Cell; Taylor and Francis Group, page 134
  12. Lesk A.M. Introduction to Protein Science, architecture, function and genomics. 3rd ed. Oxford. Oxford University Press. 2015

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