Active transport- Group Translocation, Iron Uptake

 Group Translocation

In active transport, solute molecules move across a membrane without modification. Many procaryotes also take up molecules by group translocation, a process in which a molecule is transported into the cell while being chemically altered. The best-known group translocation system is the phosphoenolpyruvate: sugar phosphotransferase system (PTS). It transports a variety of sugars into procaryotic cells while phosphorylating them using phosphoenolpyruvate (PEP) as the phosphate donor.

 PEP + sugar (outside) → pyruvate + sugar -P (inside)

 The PTS in E. coli and Salmonella typhimurium,  consists of two enzymes and a low molecular weight heat-stable protein (HPr).

l  HPr and enzyme I (EI) are cytoplasmic.

l  Enzyme II (EII) is  composed of three subunits or domains- EIIA  is cytoplasmic and soluble. EIIB also is hydrophilic but frequently is attached to EIIC, a hydrophobic protein that is embedded in the membrane.

 

Group Translocation: Bacterial PTS Transport

1. A high-energy phosphate is transferred from PEP to enzyme II with the aid of enzyme I and HPr .

2. A sugar molecule is phosphorylated as it is carried across the membrane by enzyme II.

 Enzyme II transports only specific sugars and varies with PTS, whereas enzyme I and HPr are common to all PTSs. PTSs are widely distributed in procaryotes.

 Except for some species of Bacillus that have both glycolysis and the phosphotransferase system, aerobic bacteria seem to lack PTSs.

Members of the genera Escherichia, Salmonella, Staphylococcus, and other facultatively anaerobic bacteria have phosphotransferase systems;

Some obligately anaerobic bacteria (e.g., Clostridium) also have PTSs. 

Many carbohydrates are transported by these systems. E. coli takes up glucose, fructose, mannitol, sucrose, N- acetylglucosamine, cellobiose, and other carbohydrates by group translocation.

 Iron Uptake 

Almost all microorganisms require iron for use in cytochromes and many enzymes. Iron uptake is made difficult by the extreme insolubility of ferric iron (Fe³⁺) Little free iron is available for transport. Many bacteria and fungi have overcome this difficulty by secreting siderophores [Greek for iron bearers]. 

Siderophores are low molecular weight molecules that are able to complex with ferric iron and supply it to the cell. These iron-transport molecules are normally either hydroxamates, phenolates or catecholates. Ferrichrome is a hydroxamate produced by many fungi; Enterobactin is the catecholate formed by E. coli.

Microorganisms secrete siderophores when little iron is available in the medium. Once the iron-siderophore complex has reached the cell surface, it binds to a siderophore-receptor protein. Then the iron is either released to enter the cell directly or the whole iron-siderophore complex is transported inside by an ABC transporter. After the iron has entered the cell, it is reduced to the ferrous form (Fe²⁺).

Iron is so crucial to microorganisms that they may use more than one route of iron uptake to ensure an adequate supply.

l  Electrogenic transport- Done by transport proteins/carriers that generate voltage across a membrane by the transfer of a charge, e.g., Na+/K+ATPase exchanges 3Na + for 2K + 

        A proton pump (H+) is the major electrogenic pump in plants, bacteria, and fungi. - Voltages created by electrogenic pumps are sources of potential energy available to do cellular work.

l  Electroneutral transport- where carriers exchange an equal number of charged particles or transport uncharged molecules

l  Porins – channels to transport larger molecules eg., aquaporin- allows water to cross the plasma membrane

l  Permeases – function more like an enzyme. Binds the substrate and then undergoes a conformation change which causes the carrier to release the substrate to the other side. Ex. lactose permeases