Carrier protein/transporter

Albumin is the most abundant (about 55%) of the plasma proteins. An important function of albumin is to bind with various molecules in the blood and serve as a carrier protein, transporting these substances throughout the circulation. Substances that bind with albumin include hormones amino acids fatty acids bile salts and vitamins. Albumin also serves as an osmotic regulator. Because capillary walls are impermeable to plasma proteins, these molecules exert a powerful osmotic force on water in the blood. In fact, the plasma colloid osmotic pressure exerted by plasma proteins is the only force that retains water within the vascular compartment and therefore maintains blood volume (see Chapter 15). Albumin is synthesized in the liver. 

Carrier Proteins Transport Solutes Across Biological Membranes... 

The carrier hauls the solute across. The carrier protein transports the solute from the left to the right side of the membrane. 

Steps 1-2 of Figure 29.5 Acyl Transfers The starting material for fatty-acid synthesis is the thioesteT acetyl CoA, the ultimate product of carbohydrate breakdown, as we ll see in Section 29.6. The synthetic pathway begins with several priming reactions, which transport acetyl CoA and convert it into more reactive species. The first priming reaction is a nucleophilic acyl substitution reaction that converts acetyl CoA into acetyl ACP (acyl carrier protein). The reaction is catalyzed by ACP transacyla.se. 

Molecules that cannot pass freely through the lipid bilayer membrane by themselves do so in association with carrier proteins. This involves two processes— facilitated dififrision and active transport—and highly specific transport systems. 

Facilitated diffusion and active transport share many features. Both appear to involve carrier proteins, and both show specificity for ions, sugars, and amino acids. 

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

Figure 6 Anandamide metabolism NAPE, N-arachidonylphosphatidyl-ethanol-amides PLD, phospholipase D AEA, anandamide AC, anandamide carrier protein AT, anandamide transporter AEAase, anandamide amidase AA, arachidonic acid.

Several types of cells are equipped with carrier proteins to transport essential nutrients such as glucose and amino acids that cannot cross the plasma membrane freely because of their hydrophilicity. Intestinal and renal epithelia have long been known to possess specialized Na+ cotransport processes for glucose [205], amino acids [206], and di- and tripeptides [207],... 

In the process of mediated transport, carrier proteins embedded within the plasma membrane assist in the transport of larger polar molecules into or out of the cell. When a given substance attaches to a specific binding site on the carrier protein, the protein undergoes a conformational change such that this site with the bound substance moves from one side of the plasma membrane to the other. The substance is then released. Mediated transport displays three important characteristics influencing its function ... 

Transport in blood Bound to carrier proteins Soluble in plasma Bound to carrier proteins Soluble in plasma... 

LBPs are likely to have conventional roles in the energy metabolism and transport of lipids in nematodes for membrane construction, etc. Many parasitic helminths have deficiencies in the synthesis of some lipids and so their lipid acquisition, transport and storage mechanisms clearly need to be specialized and therefore pertinent to the host-parasite relationship (Barrett, 1981). From a practical point of view, lipid transporter proteins may also be important in the delivery of anthelmintic drugs to their target most anthelmintics are hydrophobic and if they do not distribute to their site of action within the parasites by simple diffusion across and along membranes, then the parasite s own carrier proteins may be involved. 

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