Group Translocation:
* In group translocation , the transported molecule is chemically modified. or in Group translocation , molecules are transported across the cell membrane with chemical modification.
* ex. A sugar, for instance is taken up as such and is delivered inside the cell as sugar phosphate.
* Glucose, fructose, mannose & other carbohydrates are taken up by the phosphoenol pyruvate dependent posphotransferase system (PTS).
* The PTS catalyses the phosphorylation of several sugars.
* The Unique features of this system are that: i. The phosphoryl donor is not nucleotide triphosphate but phosphoenol pyruvate (PEP). ii. The system is very complex, involving many proteins.
* Group translocation involves solute modification. When the solute is modified, its exit by the same solute specific carrier is prevented.
* Group translocation is not strictly active transport, which requires unaltered accumulation of solute.
* Four proteins are known to take part in group translocation.
* Two of these are enzymes (Enzyme I and II) and Two are substrates (Hpr & Factor III)
* E- II is an integral membrane protein; it forms the channel & it catalyses the phosphorylation of sugar.
* The phosphate group, however is not directly transferred from PEP, but is first donated by E- I to a small heat stable protein called Hpr.
* The phosphorylated form of Hpr (Hpr -P) reacts with E - III which is a peripheral membrane protien.
* The phosphate group is then transferred to sugar by the channel protein - E- II (E-II & E- III are specific for each sugar).
* Whereas Enzyme I (E-I) & HPr take part in all PTS mediated sugar transport (translocation).
* A few sugar transport PTS system do not seem to involve enzyme -III.
