12 - substrates phosphate transporters

The hexose-6-phosphate transporter UhpT protein also contains 12 transmembrane (TM) regions. Based on experimental data, Hall and Maloney [113] conclude that TM11 spans the membrane as an a-helix with approximately two-thirds of its surface lining a substrate translocation pathway. It is suggested that this feature is a general property of carrier proteins in the Major Facilitator Superfamily, and that, for this reason, residues in TM11 will serve to carry determinants of substrate selectivity [113]. 

The 357-residue mammalian glucose-6-phospha-tase plays an important role in metabolism (Chapter 17). Defects in the enzyme cause a glycogen storage disease (Box 20-D) and severe disruption of metabolism.731 However, the molecular basis of its action is not well-known. Furthermore, the active site of the enzyme is located in the lumen of the endoplasmic reticulum732 and glucose-6-phosphate must pass in through the plasma membrane. An additional glucose-6-phosphate transporter subunit may be required to allow the substrate to leave the cytoplasm.73... 

The pyruvate, glutamate and phosphate transporters catalyze net uptake and release of their substrates with stoicheiometric amounts of protons [6]. Early evidence for the electroneutrality of the process was the good inverse correlation between the H gradient across the mitochondrial membrane and the gradients of these permeant anions, especially at equilibrium and at low metabolite concentrations [96,97]. At equilibrium the rate of inward transport should equal the rate of efflux and the distribution of permeant anion should be proportional to the A pH since ... 

A particularly reactive sulfhydryl group (or groups) on the phosphate transporter react with relatively low concentrations of maleimide derivatives and organic mercurials to inhibit the function of the transporter [17]. This sulfhydryl group is probably near, or a part of, the substrate binding site because phosphate protects the carrier activity from inhibition by A-ethylmaleimide. Fonyo has written a comprehensive review on the subject of sulfhydryl sensitivity of the phosphate carrier and its implications concerning the mechanism of transport [192]. 

Lemieux, M.J., Huang, Y., and Wang, D.N. (2004) The structural basis of substrate translocation by the Escherichia coli glycerol-3-phosphate transporter a member of the major facilitator superfamily. Current Opinion in Structural Biology, 14 (4), 405-412. 

Figure 1 Bacterial reduction of arsenate and oxidation of arsenite. (A). Cytoplasmic arsenate reductase (ArsC) as encoded by bacterial ars operons along with chromosomaUy encoded Pit and Pst phosphate transport systems with arsenate as an alternative substrate. After reduction from arsenate to arsenite, arsenite is removed from the cell by the ArsB membrane protein. (B). Anaerobic periplasmic arsenate reductase. (C). Aerobic periplas-mic arsenite oxidase, hnked via azurin to the respiratory chain.

Fig. 3. Primary carbon metabolism in a photosynthetic C3 leaf. An abbreviated depiction of foliar C02 uptake, chloroplastic light-reactions, chloroplastic carbon fixation (Calvin cycle), chloroplastic starch synthesis, cytosolic sucrose synthesis, cytosolic glycolysis, mitochondrial citric acid cycle, and mitochondrial electron transport. The photorespiration cycle spans reactions localized in the chloroplast, the peroxisome, and the mitochondria. Stacked green ovals (chloroplast) represent thylakoid membranes. Dashed arrows near figure top represent the C02 diffusion path from the atmosphere (Ca), into the leaf intercellular airspace (Ci), and into the stroma of the chloroplast (Cc).SoHd black arrows represent biochemical reactions. Enzyme names and some substrates and biochemical steps have been omitted for simplicity. The dotted line in the mitochondria represents the electron transport pathway. Energy equivalent intermediates (e.g., ADP, UTP, inorganic phosphate Pi) and reducing equivalents (e.g., NADPH, FADH2, NADH) are labeled in red. Membrane transporters Aqp (CO2 conducting aquaporins) and TPT (triose phosphate transporter) are labeled in italics. Mitochondrial irmer-membrane electron transport and proton transport proteins are labeled in small case italics.

The transport of each COg requires the expenditure of two high-energy phosphate bonds. The energy of these bonds is expended in the phosphorylation of pyruvate to PEP (phosphoenolpyruvate) by the plant enzyme pyruvate-Pj dikinase the products are PEP, AMP, and pyrophosphate (PPi). This represents a unique phosphotransferase reaction in that both the /3- and y-phosphates of a single ATP are used to phosphorylate the two substrates, pyruvate and Pj. The reaction mechanism involves an enzyme phosphohistidine intermediate. The y-phosphate of ATP is transferred to Pj, whereas formation of E-His-P occurs by addition of the /3-phosphate from ATP ... 

Rotenone inhibits the transfer of electrons from NADH into the electron transport chain. The oxidation of substrates that generate NADH is, therefore, blocked. However, substrates that are oxidized to generate FADH2 (such as succinate or a-glycerol phosphate) can still be oxidized and still generate ATP. Because NADH oxidation is blocked, the NADH pool becomes more reduced in the presence of rotenone since there s nowhere to transfer the electrons. 

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