Synthesis, Transport, and Metabolism

Schubert, K.R. (1986). Products of biological nitrogen fixation in higher plants synthesis, transport, and metabolism. Annual Review of Plant Physiology 37, 539-74. 

Structure of a proteoglycan. [Reproduced with permission from J. E. Silbert and G. Sugumaran, Intracellular membranes in the synthesis, transport, and metabolism of proteoglycans. Biochim. Biophys. Acta 1241 372 (1995).]... 

A group of proteins known as globular proteins have compact, spherical shapes because sections of the polypeptide chain fold over on top of each other dne to the various interactions between R groups. It is the globular proteins that carry ont the work of the cells functions such as synthesis, transport, and metabolism. 

Iron is another vital nutrient in the development of functioning erythrocytes it is essential for the formation of hemoglobin. Lack of iron leads to a decrease in hemoglobin synthesis and ultimately red blood cells. Normal homeostasis of iron transport and metabolism is depicted in Fig. 63-2.7 Approximately 1 to 2 mg of iron is absorbed through the duodenum each day, and the same amount is lost via blood loss, desquamation of mucosal cells, or menstruation. 

Enzymes associated with myelin. Several decades ago it was generally believed that myelin was an inert membrane that did not carry out any biochemical functions. More recently, however, a large number of enzymes have been discovered in myelin [37]. These findings imply that myelin is metabolically active in synthesis, processing and metabolic turnover of some of its own components. Additionally, it may play an active role in ion transport with respect not only to maintenance of its own structure but also to participation in ion buffering near the axon. 

The primary developmental mechanism of the atherosclerotic process is not completely understood. It seems likely that the development of atherosclerosis is preceded by metabolic abnormalities of the synthesis, transport, and utilization of lipids. Lipids such as triglycerides and cholesterol esters are circulated in the blood in the form of particles (lipoproteins) wrapped in hydrophilic membranes that are synthesized from phospholipids and free cholesterol. Cholesterol is transported by particles of various sizes synthesized from triglycerides, cholesterol esters, and phospholipids, each of which plays a very specific role. 

In the human body choline is needed for the synthesis of phospholipids in cell membranes, methyl metabolism, transmembrane signaling and lipid cholesterol transport and metabolism [169]. It is transported into mammalian cells by a high-affinity sodium-dependent transport system. Intracellular choline is metabolized to phosphorylcholine, the reaction being catalyzed by the enzyme choline... 

Although several lipoproteins are considered to play a role in atherogenesis [VLDL, LDL and Lp(a)j, LDL cholesterol (LDL-C) is the primary target of therapy. The risk of CHD is inversely related to levels of HDL, because HDL is responsible for reverse cholesterol transport. Lipoprotein disorders can involve abnormalities in lipid metabolism (e.g., synthesis, transport, and catabolism). Attainment of a lipid profile must be made after a 9- to 12-hour fast. 

Abstract The cannabinoid neurotransmitter system comprises cannabinoid G protein-coupled membrane receptors (CBi and CB2), endogenous cannabinoids (endocannabinoids), as well as mechanisms for their synthesis, membrane transport and metabolism. Within the brain the marijuana constituent -tetrahydrocannabinol (THC) produces its pharmacological actions by acting on cannabinoid CBl receptors. THC modulates neuronal excitability by inhibiting synaptic trans-... 

This report has briefly summarized what we now know about thyroid hormone transport to the central nervous system. The data are still sketchy and much remains to be done. Obviously the brain is a complex organ and major differences in thyroid hormone transport and metabolism are to be expected in its constituent parts, so study of different brain regions as well as different cell types is required. We also need to distinguish between findings in the mature brain and those during fetal and postnatal development. Thyroid hormones play a very different role in these stages of the organism, and possible variations in hormone delivery to cells may contribute to these differences. Finally, in the malnutrition that often accompanies iodine deficiency, we need to ask whether PA synthesis in the choroid plexus is compromised, as it is in the liver. If so, important effects in thyroid hormone delivery to the brain may be expected. 

Dubach, R., Moore, C.V., and Minnich, V. (1946) Studies in iron transportation and metabolism V. Utilization of intravenously injected radioactive iron for hemoglobin synthesis, and an evaluation of the radioactive iron method for studying iron absorption./. Lab. Clin. Med., 31, 1201-1222. 

In 1980, Lea and Miflin discussed the transport and metabolism of asparagine and other nitrogen compounds within the plant. Since that time there have been major developments in the study of asparagine breakdown but the precise enzymology of asparagine synthesis in green leaves has still not been established (Sieciechowicz et al, 1988a). 

This potential, or protonmotive force as it is also called, in turn drives a number of energy-requiring functions which include the synthesis of ATP, the coupling of oxidative processes to phosphorylation, a metabohc sequence called oxidative phosphorylation and the transport and concentration in the cell of metabolites such as sugars and amino acids. This, in a few simple words, is the basis of the chemiosmotic theory linking metabolism to energy-requiring processes. 

The synthesis and metabolism of DA are very similar to that of NA, even when it functions as a NT in its own right. Although both phenylalanine and tyrosine are found in the brain it is tyrosine which is the starting point for NA and DA synthesis. It appears to be transported into the brain after synthesis from phenylalanine (phenylalanine hydroxylase) in the liver rather than from phenylalanine found in the brain. Despite the fact that the concentration of tyrosine in the brain is high (5 X 10 M) very little body tyrosine (1%) is used for the synthesis of DA and NA. 

Figure 6.1 Histamine synthesis and metabolism in neurons. L-histidine is transported into neurons by the L-amino acid transporter. Once inside the neuron, L-histidine is converted into histamine by the specific enzyme histidine decarboxylase. Subsequently, histamine is taken up into vesicles by the vesicular monoamine transporter and stored there until released. In the absence of a high-affinity uptake mechanism in the brain, released histamine is rapidly degraded by histamine methyltransferase, which is located postsynaptically and in glia, to telemethylhistamine, a metabolite that does not show any histamine-like activity.

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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