Protein

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. 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 humans dry bodyweight. 

Structure

A protein has several 'layers' of structure . The function of the protein is determined by its structure, therefore each layer is dependent on the next.

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 know as 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 for the peptide backbone.

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 the amino acid Cysteine .

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 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 .

Functions of Proteins

Proteins make up 50% of each cell and have both structural and functional importance. Proteins transport numerous 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:

• the silk-beta pleated sheet which has Alanine and Glycine residues forming a rigid, stable structure. Spiders can make silk, and in this type of silk the rigid sections alternate with stretchy ones in order to make the structure both strong and elastic.
• A-Keratin which is present in hair, nails, and wool (among others). This structure is usually stretchy and flexible, however when many disulphide bridges are present (for example, in hooves and nails) the structure remains rigid and loses flexibility. 
• Collagen, consisting of a coil of three strands of glycine-proline-proline which is 100 strands long. This is the most abundant protein in mammals.

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 .

Synthesis of Proteins 

Proetin syntheiss can be divided into two sections, transcription and translation. In transcription DNA is used to code for the protein, it start 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 complementarty sequnces 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 sequnces of mRNA. This mRNA is used to code for the protein sequence.   

In transaltion the the mRNA binds to a ribosome, this ribosme then moves down the mRNA from the 5' to 3' end. tRNA has an anticodon sequnce 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 tripplet code on the mRNA. Peptide bonds are formed between amino acids next to each other ( when their two triplet codes are next to ecah other) . 

This forms the primary structure of proteins which is the amino acid sequence. 

See also

• Amino acid