Membrane Transport

Membrane Transport

ABSTRACT

The phospholipid bilayer is vital for maintaining cellular function and survival. The hydrophobic and hydrophilic nature of the lipid monomer confers selective permeability for many biological molecules. Transport proteins located within the phospholipid bilayer primarily act to achieve transport of solutes incapable of simple diffusion. Transport proteins are comprised of two general classes, carriers or channels, depending on their differential mechanism of action. Carrier proteins posit the capacity to cycle between conformational changes, by coupling movement of ions or molecules with or against respective concentration gradients, illustrating scenarios of either passive or active transport. There are hundreds of superfamilies of transport proteins belonging to the biological membrane, each giving rise to specific reaction equations and structural features. The largest is the major facilitator superfamily, distributed vastly among bacteria, archaea, and eukarya. The Major Facilitator Superfamily demonstrates a generalized transmembrane helices conformation, with all members sharing the common Major Facilitator Superfamily fold. Prominent conformational changes via movement in the transmembrane segments comprise the alternating access transport mechanism, essentially characterizing this superfamily as secondary transporters. The other large superfamily is the ATP-Binding Cassette transporter superfamily, which utilizes chemical energy generated by ATP hydrolysis, characteristic of primary active transporters. Transportation of molecules by ATP-Binding Cassette members occurs in a thermodynamically unfavorable direction. Alike the Major Facilitator Superfamily, ATP-Binding Cassette proteins also exhibit the alternating access model. A group of transporters of increasing medical interest is the multi-drug resistant transporters, which utilizes either the protein gradients or ATP as efficient sources of energy. In any case, membrane transport proteins exhibit a high degree of specificity for the substance transported, with the rate of transport differing considerably owing to difference in the mechanism of action.

MECHANISMS OF MEMBRANE TRANSPORT

Objectives

Describe the difference between a carrier protein and a channel in terms of conformational changes.

Identify the direction of solute transport for each type.

The fatty acid tails of lipid monomers comprising the hydrophobic interior of the biological membrane poses as a substantial barrier for simple diffusion. Charged, polar and bulky molecules are not able to diffuse directly from one side of the membrane to the next. In this case, membrane transport proteins specialized for such non-diffusible molecules permit transmembrane movement. Transport proteins are recognized by a slew of different names, depending somewhat arbitrarily on their mode of action: transporters, translocases, permeases, pores, channels, and pumps, just to list a few. Further, transport proteins may be classified by the substance that they transport, whether they are always open or gated (open only when stimulated), and most importantly, whether a source of free energy is required for operation.