Introduction:The term structure refers to the construction of something ina 3-dimensional space made from the interactions of its components. In biology,these objects may refer to proteins which are made up of amino acids monomers andthe interactions between these amino acids include hydrogen bonds anddisulphide bridges.Function refers to the purpose or role of something in itsenvironment. Within biology, this may refer to a process as a part of a largersystem or body arrangement.
The term protein, as briefly mentioned previously, refersto a structure that is made up of amino acid monomers with specificintramolecular interactions that allow it to form a 3D shape. Which as a resultcauses it to have a distinctive function in a larger system.The protein that will be explored and discussed in thisessay will be the Cholera Toxin.Briefbackground of the Cholera Toxin: The cholera toxin was first identified in 1959 by Prof.Sambhu Nath De at Kolkata (De, 1951)1. Itis a protein complex that is responsible for the cholera infection. It isreleased by the bacteria, Vibrio cholerae.
Structureof the Cholera Toxin:The structure of the cholera toxin is an oligomeric one.The term oligomer, dissimilar to a polymer, refers to the structure of aprotein being made up of a couple individual monomers compared to the vastnumber monomers which make up a polymer. Oligomers are usually made up of 2-6monomers (Wilkinson, 1997)2.The cholera toxin is made up of 6 different monomers (protein subunits), 5 ofthe 6 units belonging to the receptor binding component (5 B subunits) of theprotein and the final unit belonging to the part (1 A subunit) which isresponsible for metabolic reaction. Figure 1 shows the 5 B subunits of thecholera toxin, and their pentagonal arrangement. The enzymatic subunit is not shown in Figure 1; however, itis shown in Figure 2.
But we can see the side view of the pentagonal structureof the 5 B subunits. Also, on top of that, the 1 A subunit. Altogether, thesetwo subsections make up the cholera toxin’s structure. The piece of proteinthat joins these two subunits is a simple alpha helix which has a peptide linkat either end to keep the molecule as one unit. Collectively, it becomes an AB5complex. The A subunit is divided into two smaller subsegments, A1 (also known as CTA1 & CTA2 respectively) which is held together by adisulphide bond (shown by a red circle in figure 3). This will be importantwhen discussing how structure of the toxin relates to the function. Functionof the Cholera Toxin: The overall function of the cholera toxin is to trigger theCFTR (Cystic fibrosis transmembrane conductance regulator) to release morechloride ions into the apical side of the membrane.
When the Vibrio Cholerae releases the Cholera toxin, itinteracts with the GM1 Ganglioside (by receptor-binding site interactions).This causes the release of the A1 subunit into the Epithelial cell – this isdone via the breaking of the disulphide bond. This G-protein can now bind to theAdenylate Cyclase which causes the production of the cAMP (cyclic AMP) whichthen activates a PKA (protein kinase A). This then phosphorylates the CFTRprotein causing a greater concentration of chloride ions to be released.
As aresult, Na+ follows the Cl- due to electrostaticattraction, which then further causes water to follow due to the overall electrochemicalgradient. This causes an excess of water in apical membrane (of the intestinecells) which leads to the organism who has been infected by the toxin to havewatery excrements.Importance of structure on the function ofthe Cholera toxin The structure of the cholera toxin is crucially importantto its function. Having mentioned before that is has 5 subunits, it’s essentialfor the actions that take place in the epithelial cells that the processing andfolding of the protein is done to perfection. Shown in figure 5 is the 5receptor binding sites of the cholera toxin. Although they look very similarinitially, we can see that the coloured areas (indicating the differentelectron density maps of each binding site) are different at key points of eachsite.
This indicates that distinctive areas of the receptor binding sites willinteract in a contrasting way to other areas (via electron overlaps). Thisproves the influence of the structure of the function of the toxin. Receptor-toxinbinding sites must be totally complimentary for the whole molecule tosuccessfully activate the function of the toxin. Hence the relevance of the 3-dimensionalstructure.Furthermore,as well as the initial interaction between the toxin’s B subunits and theGM1-Ganglioside cell surface sphingolipid, the A subunit of the cholera toxinis released into the epithelial cell. For this to occur, it must be small yetaccurately structured to allow it to pass the membrane. If the structure is toobig to fit through the membrane, then the action of the toxin is stopped. Notonly this but as mentioned before there are two subsegments of the A subunit,which are joined by a disulphide bond.
If there was twice as many disulphidebonds then the release of the A1 subsection may not be possible which will meanthe process shown in figure 4 will be halted, as nothing will be able toactivate the G-protein. On the other hand, if in place of the disulphide bondthere was a hydrogen bond (which is much weaker), the resulting effect will bethat the bond might break on its way to the epithelial cell, which will thenmean although the five B subunits will attach onto the GM1 ganglioside, therewill be no A1 subunit present to release into the cell, hence having no onseteffect on the CFTR protein. Conclusion: To conclude, the structure of the cholera toxin isessential in its function. With many components that make up the toxin’sstructure it is vital that everything works in unison – most significantly with5 receptor binding sites of the toxin.References1. De, S.
N. et al. (1951). An experimental study of the action of choleratoxin. The Journal of Pathology. 63 (1), 707-717.
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