Solute-binding protein-dependent transport is called a shock-sensitive transport system. A Variety of nutrients including Sugars, amino acids and ions can be transported through the shock-sensitive transport system. solutes cross the outer membrane through porins and bind specific binding proteins before being transported through the cytoplasmic membrane by a membrane-bound protein complex. This protein complex is a member of a large superfamily of proteins that import nutrients or export cell surface constituents and extracellular proteins (Section 3.8.2). They have an ATP-binding motif and hydrolyze ATP to supply energy for the transport. They are called ATP-binding cassette (ABC) transporters and are known in all organisms
A glutamatebinding protein is known in Rhodobacter sphaeroides, but glutamate transport is driven by the electrochemical gradient in this bacterium. This system is known as the TRAP (tripartite ATP-independent periplasmic) transporter. TRAP transporters are shock insensitive.
Through active transport, nutrients are accurrrulated in the cell with the expenditure of a large amonnt of energy in the forrn of the electrochemical gradlent or ATP. Mlcrobes can grow efficiently in environments with low nutrient concentrations due to active trans- port systems. Active transport can be summarized as follows:
(1) Carrier proteins have solute specificity as in the enzyme- substrate relationship.
(2) Energy is needed to change the affinity of the transporter for the transported solute at the other side of the membrane.
(3) The transported solute can be accumulated against a concentra- tion gradient.
(4) The structure of the solute does not change during active transport.