Question

Explain why cysteine residues with free sulfhydrl groups tend to be localized on the exterior of a protein molecule, wheras those involved in disulfide bonds are more likely to be buried in the interior of the molecule?

How can two polymers consisting of the same repeating subunit have such different properties? Explain in great detail.

Answer 1

Exterior sulfhydryl groups of cysteine present in protein act as a strong reducing factor that helps in maintenance of redox potential at protein microenvironment (ex. Glutathione, an oligo peptide) that require for proper biological functions at certain compartments in vivo. This SH group of cysteine functions as a scavenger of free radicles, where the 'H' will be replaced by 'R' group (any molecule except hydrogen). It also involves in detoxification of drugs via glutathione that has cysteine residue. Moreover, enzymes (thiol proteases) consists cysteine at their active sites for binding with a specific substrate.

For example, starch and glycogen are the polymers (polysaccharides) alpha-D-glucose. Glycogen is major energy reservoir in animals that stores in liver and starch is the major energy reserve in plants. Both are made up of (amylose) glucose residues with alpha-1-4-glycosidic linkage with a straight chain. But they difference in the branching (amylopectin) points; in glycogen for every 8-12 glucose residues there will be a branch that is with alpha-1-6-glycosidic linkage. Whereas, in starch it repeats for every 24-30 glucose residues.

Cellulose is also a polymer of glucose but, differ in bonding compared to starch containing amylose. It is with beta-1-4-glycosidic linkage and most abundant organic compound on Earth. It usually present in all parts of plants. This subtle difference in structure it cannot be digested by us since we do not have cellulase enzyme to digest this.

Chitin is also glucose polymer, structurally resemble with cellulose but abundantly present in the exoskeleton of arthropods and fungi cell walls.

Next, if we look at DNA or mRNA both are polymers of nucleotides that are A, G, C, T in DNA and A, G, C, U in RNA This existence is universal. But the polymerisation of these nucleotides in prokaryotes and eukaryotes associated with different physical and biochemical properties.The absence of introns, splicing mechanism, polycistronic mRNA and coding for 70S ribosomes and so, differ in prokaryotes compared to eukaryotes and presence of plasmids, a circular extrachromosomal DNA in prokaryotes that absent in eukaryotes.