Transport of nutrients by bacteria - Active Transport

Diffusion and facilitated diffusion can efficiently move molecules to the interior when the solute concentration is higher on the outside of the cell, they cannot take up solutes that are already more concentrated within the cell (i.e., against a concentration gradient). Microorganisms often live in habitats characterized by very dilute nutrient sources, and, they must be able to transport and concentrate these nutrients against a concentration gradient.

Microbes use two important transport processes in such situations: active transport and group translocation. Both are energy-dependent processes.

Active transport is the transport of solute molecules to higher concentrations, or against a concentration gradient, with the use of metabolic energy input. Because active transport involves protein carrier activity, it resembles facilitated diffusion in some ways. The carrier proteins or permeases bind particular solutes with great specificity for the molecules transported.

Similar solute molecules can compete for the same carrier protein in both facilitated diffusion and active transport. Active transport is also characterized by the carrier saturation effect at high solute concentrations

However, active transport differs from facilitated diffusion in its use of metabolic energy and in its ability to concentrate substances. Metabolic inhibitors that block energy production will inhibit active transport but will not affect facilitated diffusion (at least for a short time).

 

1. ATP driven Binding protein transport

Binding protein transport systems or ATP-binding cassette transporters (ABC transporters) are active in bacteria, archaea, and eukaryotes. Usually these transporters consist of two hydrophobic membrane-spanning domains. These are associated with two nucleotide-binding domains on their cytoplasmic surfaces . 

The membrane-spanning domains form a pore in the membrane and the nucleotide-binding domains bind and hydrolyze ATP to drive uptake.

ABC transporters use substrate binding proteins, which are located in the periplasmic space of gram-negative bacteria or are attached to membrane lipids on gram-positive plasma membrane. These binding proteins, bind the molecule to be transported and then interact with the membrane transport proteins to move the solute molecule inside the cell. 

                ABC Transporter Function.

(1) The solute binding protein binds the substrate to be transported and approaches the ABC transporter complex.

(2) The solute binding protein attaches to the transporter and releases the substrate, which is moved across the membrane with the aid of ATP hydrolysis.

E. coli transports a variety of sugars (arabinose, maltose, galactose, ribose) and amino acids (glutamate, histidine, leucine) by this mechanism.

Substances entering gram-negative bacteria must pass through the outer membrane before ABC transporters and other active transport systems can take action. There are several ways in which this is done. 

• When the substance is small, a generalized porin protein such as OmpF can be used
• larger molecules require specialized porins. 
• In some cases (e.g., for uptake of iron and vitamin B12), specialized high-affinity outer membrane receptors and transporters are used.

 

 Eucaryotic ABC transporters are sometimes of great medical importance. Some tumor cells pump drugs out using these transporters. Cystic fibrosis results from a mutation that inactivates an ABC transporter that acts as a chloride ion channel in the lungs.