The human genome is estimated to contain between 20,000 and 25,ooo protein coding genes, however the number of proteins present within the human body is far greater that this. Within humans and other organisms a variety of proteins must therefore be expresses from the same gene. This is a result of alternative RNA splicing and post translational modification.
Alternative RNA splicing involves different mRNA molecules being produced from the Sam eprimary transcript depending on which RNA segments are treated as exons and introns. Exons are the parts of the gene which codes for protein whereas introns are non coding parts of the gene. In Drosophilia melanogaster the dsx gene undergoes alternative splicing in the embryo to determine the organism’s gender. In male exons 1,2,3,4,5 and 6 are combined whereas in females it is exons 1,2,3 and 4.
The hormone insulin, which increases the uptake of glucose by cells, consists of two polypeptide chains, which originate as one chain. Disulphide bridges form between cysteine residues in the original polypeptide chain, known as pro-insulin. A protease enzyme (an enzyme that cuts protein at a peptide bond) cuts the polypeptide chain in two places. The middle section of the protein is then removed.
A carbohydrate molecule is most commonly added to asparagine, serine or threonine to produce a glycoproteins through the process of glycosylation. Glycoproteins can preform a variety of roles and are often found as integral membrane proteins aiding cell-cell interactions, including antibody action and white blood cell recognition processes. Other examples are antifreeze proteins in cold water fish and proteins in mucus.
Proteins can also become phosphorylated with the addition of a phosphate group by a kinase enzyme. This is an important mechanism in controlling the activity of many enzymes and receptors. The addition of a phosphate group causes a conformational change in the protein structure, often switching it ‘on or off’. Alternatively this phosphorylation may change the cellular location of the protein or its association with other proteins. This is often reversible, with the phosphate group being removed by a phosphatase enzyme. Examples of this include the phosphorylation of Na+/K+ ATPase, which is involved in transporting sodium and potassium across the cell membrane.
Author – Jiangmin Hou
Jiangmin is a 5th year high school student currently studying five STEM subjects at Scottish Higher level-Mathematics, Physics, Biology, Computer Science and Chemistry. She is interested in pursuing a degree in Medicine after completion of Secondary Education.