Bacterial Cell Membranes and Proteoliposomes: Understanding Solute Transport - Prof. Ranja, Study notes of Health sciences

Download Bacterial Cell Membranes and Proteoliposomes: Understanding Solute Transport - Prof. Ranja and more Study notes Health sciences in PDF only on Docsity! Bacterial Physiology: HSCI 4607/5707 Ch: 16 Solute Transport Introduction: Bacterial cell membranes are semipermeable and made up of phospholipid matrix which blocks the diffusion of water soluble molecules into and out of cells. It separates the cell’s internal environment from the external environment facilitating metabolic activities and growth by maintaining higher concentration of metabolites inside. This promotes faster enzymatic reactions and retention of metabolic intermediates within the cells. Lipid barrier also prevents diffusion of ions including protons which help in the maintenance of proton and Sodium gradient across the membrane for the production of ATP, solute transport etc.. All water soluble molecules enter or come out of cell through integral membrane proteins including transporters, carriers, porters and permeases. Studies on membrane transport can be carried out using artificial lipid membrane system known as ‘Proteoliposomes’. Kinetics of Solute Uptake: 1. Transporter-mediated uptake: One can prove the existence of the transporter by studying the kinetics of the solute transport. The kinetic studies are carried out by measuring solute transport at various concentration of solute and plotting the graph of transported solute versus different solute concentrations. The graph appears to be a hyperbolic in case of the presence of specific transporter. Km [S] Vmax v Vmax/2 Transporter-mediated uptake………. The explanation for the hyperbolic curve is that the entry through the transporters is always limited by the numbers of transporters present in the membranes. Thus initially the numbers of the transporters available are more and as the concentration of solute in the medium progressively increases, all the transporters get occupied the transport reaches to a maximum velocity (Vmax). The solute concentration at half the maximum velocity is called as Km. Km, a affinity constant, is frequently used to express the affinity of the solute towards the transporter. The value of Km for different transporter vary from less than micromolar to several hundred micromolar. 2. Uptake in absence of transporter: When the same concentration dependent experiment is carried out for transporter-independent solute transport, following type of graph is generated. v [S] As seen in the graph the rate of transport is much slower and is proportionate to the concentration of solute. The transport does not get saturated even at higher concentration of solute. Secondary Transport: Generally, the secondary solute transporter functions in such a way that either both solute and ion are transported in one direction (Symport) or the solute in one direction and the ion in the other (antiport). But there are transporters that transport just an ion along its electrochemical gradient called uniport. In all the cases these transporters are the part of an electrical circuit. Most of the bacteria use both the proton as well as sodium symporters but mainly the former. But certain bacteria like halophiles, alkaliphiles, and marine bacteria depend heavily on sodium symporters. The examples of transports coupled with the electrochemical gradient are: 1. Symport of an uncharged solute with protons; 2. Symport of a monovalent anion with protons; 3.Antiport of a monovalent cation with H+; and 4. Electrogenic uniport of a cation. Evidence for solute/Proton or solute/sodium symport: One way to demonstrate coupling of transport to proton or sodium ion influx is to measure the alkalinization of the medium (decrease in protons) or a decrease in the Na+ ion concentration when bacteria or membrane vesicles are incubated with the appropriate solutes. During Na+/H+ symport the Na+ or H+ can be measured by specific ion sensitive electrodes. ATP-driven Primary transport: There are many transport systems that are driven by ATP or some phosphorylated derivatives. The examples of such transport are: 1. H+ transport (ATP synthase); 2. K+ transport in E.coli.; 3. Some transport systems in gram positive bacteria; 4. Transport systems in gram-negative bacteria that uses periplasmic binding proteins. Shock-sensitive transport systems: These systems are the characteristics of gram-negative bacteria and are responsible for the transport of wide range of solutes, including sugars, amino acids, and ions. Usually these systems consist of a transporter-a inner membrane protein complex consisting of four subunits (two of which are identical) and a periplasmic solute binding protein. Such systems are more complex than p driven single transporter transport. The transport process consist of several steps. They are known as shock sensitive transport system since osmotic shock leads to the loss of these systems due to the loss of the periplasmic proteins.