3.6 precursor/product antiport
Some lactic acid bacteria utilize the potential energy developed by the accumulation of fermentation product in the cell to drive nutrient transport in a similar manner to antiport. Instrad of H or Na a fermentation product is exchanged with its precursor and this system is therefore referred to as the precursor/product antiport.
A malate/lactate antiport system is known in species of lacto . they generate a proton motive force fermenting malate to lactate through the well-docomented malolactic fermentation pathway. Species of streptococcus ferment arginine to ornithine through citrul-line to produce ATP. Ornithine is exchanged with arginine in 1:1 ratio.
Lactose is imported through a H –symport system in strep. Lactose can be transported by a lactose/galactose antiport system in these bacteria when the lactose concentration is high. Lactose is hydrolyzed to glucose and galactose by beta-galactosidase. Glucose is metabolized through glycolysis and galactose is exchanged with lactose. Excreted galactose is then utilized after all the lactose is consumed.
In addition to lactic acid bacteria, precursor/product antiport systems are known in other anaerobic fermentative bacteria such as the oxalate/formate antiporter in oxa and the betaine/n,n-dimethyl glycine antiporter in eub. Fugure 3.7 summarizes some of the nutrient uptake pathways known in prokaryotes.
3.7 Ferric ion (Fe(III)) uptake
In natural aerobic ecosystems, almost all iron is present as the ferric ion (Fe(III)) since ferrous iron (Fe(II)) is auto-oxidized with molecular oxygen at neutral pH. Ferric iron is virtually insoluble in water with a solubility of around 10 M and this is much lower than the 10 M necessary to supply adequate iron for most microbes. To overcome this problem, many microbes synthesize and excrete low molecular weight ferric iron chelating compounds known as siderophores for the sequestration. Siderophores form complexes with ferric iron that are imported into the cell by an ABC transport system. Siderophores are collection of compounds with a variety of chemical structures. Two main structural classes of siderophores are the catecholamides and the hydroxamates. A given organism may produce siderophores of one or both classes.
3.8 Export of cell surface structural components
Large numbers of proteins and polysaccharides constitute the cytoplasmic membrane, periplasm (including the cell wall), and outer membrane. many prokaryotes also secrete extracellular enzymes and toxins. These must be translocated through the cytoplasmic membrane after their synthesis in the cytoplasm. Translocation into and through the cytoplasmic membrane is referred to as protein transport. The term ‘secretion’ is used to refer to protein translocation further away from the cytoplasmic membrane to the cell surface and to the extracellular medium. Proteins are transported through one of three mechanisms. These are the general secretory pathway(GSP), the ABC pathway and the twin-arginine translocation (TAT)pathway.
The outer membrane is another barrier for protein secretion in gram-negative bacteria and several different mechanism have been identified as protein secretion pathway.