PROTEIN CHEMISTRY (CHM G222)
FINAL EXAM
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PROTEIN CHEMISTRY (CHM G222)
FINAL EXAM
Post translational processing involves any change in the protein once it has been translated in the ribosome. It includes covalent modifications and non-covalent modifications. In the cholera toxin (CT), before secretion, the signal peptide is cleaved. During assembly disulfide bond is formed between the A1 and A2 fragments of the A domain. During trafficking of the toxin, the A1 fragment is proteolytically cleaved off the protein but is held by the disulfide linkage. The disulfide linkage is the broken by a chaperone PDI.1,2
Proteins are synthesized in the cytosol. The proteins are transported to their respective destinations with the help of a signal sequence. The protein is first transported to the ER and then to other compartments. The proteins which are to stay in the ER contain an additional signal sequence at the C-terminal, KDEL. This is an ER retention signal. In cholera toxin (CT) the A2 fragment of the A subunit contains a KDEL sequence at the C-terminal. The KDEL sequence is recognized by ERD2-KDEL receptor on the Golgi. Generally this sequence is used to bring back the proteins which escape to the ER. But in CT, it is used to prevent the toxin from moving out of the ER.2,3,4,5
G proteins are important regulatory proteins found on the inner surface of the plasma membrane. They bind to both GDP and GTP and the resulting conformational changes in the proteins leads to activation of an effector molecule. CT irreversibly ADP-ribosylates Gsα, which is G protein that activates the adenylate cylase complex. It leads to increase in cAMP levels which in turn lead to a cascade of events. The net result being, a flood of sodium and water in the gut.3,6
There are many forces that help in holding a protein in its tertiary structure. The disulfide bonds are formed between two cysteine residues which are placed apart in the sequence, thus bringing together different regions of a protein. Salt bridges are formed between the positively charged basic residues and negatively charges acidic residues. Hydrogen bonds can be formed between residues having a donor (say H) and acceptor atom (say O). In CT, the A1 and A2 fragments are held together by a disulfide linkage. The entire toxin protein contains around 130 hydrogen bonds and 20 salt bridges. The B pentamer is held together by a number of hydrogen bonds. The A subunit (A1 and A2) form hydrogen bonds with the B subunit. The positive charges on the interior of the B subunit pore interact with the negative charges on the A2 fragment. 1
A number of secretory proteins are stabilized by disulfide linkages. Chaperones like PDI (Protein disulfide isomerase) help in ensuring that the protein has the correct disulfides and rearranges incorrectly introduced disulfides. Once CT enters the ER, the reduced ...
