Sugar Carrier

Longer chain isoprenoids are synthesized via two other isoprenyl diphosphate enzyme systems in mammals [21]. Long E-isoprenyl diphosphate synthase (IDS) produces the side chains of ubiquinone. The length of the side chain varies amongst species, and in humans a C50 synthase has been identified [22]. Dehydrodolichyl diphosphate synthase, the only Z-IDS found in mammals, is responsible for the synthesis of the sugar carriers dohchol and dolichyl phosphate [23]. Plants have additional Z-IDS which can catalyze the production of very long isoprene species, such as natural... 

In this system, glucose is postulated to form a complex with a carrier molecule at the outer surface of the cell. The sugar-carrier complex passes across the membrane and releases free glucose at the inner surface. The process is reversible. The maximum transport rate, Tmax, is limited by fixed properties of the system such as the total number of carriers and their movement. Below this limit, however, transport will vary with the sugar concentration since this determines the extent of complex formation according to Langmuir adsorption or Michaelis-Menten kinetics. Thus unidirectional transport into the cell can be expressed as follows ... 

Isoprenoid lipid carriers are used to convert the hydrophilic nucleotide diphosphate sugars into a form that is able to penetrate the cytoplasmic membrane. It will be recalled that isoprenoid sugar carriers are also used in the biosynthesis of peptidoglycan (p. 14) and LPS (p. 79). In actively growing bacteria there must be sufficient sugar-charged isoprenoid carrier... 

Baits include mixtures of toxicant, usually at l ndash 5%, with a carrier especially attractive to the insect pest. Carriers include sugar for the houseflies, protein hydrolysates for fmit flies, bran for grasshoppers, and honey, chocolate, or peanut butter for ants. 

Cocoa butter is the common name given to the fat obtained by subjecting chocolate Hquor to hydrauHc pressure. It is the main carrier and suspending medium for cocoa particles in chocolate Hquor and for sugar and other ingredients in sweet and milk chocolate. 

Oxalic Acid.—The jurciiaration of ovalic acid by the action ol nitric aiid on sugar was introduced bySchccle, and was used for some time as a technical process. I hc vanadium pent-o. ide aits as carrier of o ygen, being alternately leduccd to tctro. ide and re-o. idised. The picscnt commercial method is to heat sawdust with a mi. cturc of caustic potash and soda on Cohen s adv. p. o. c. ... 

The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biological polymers that act as chemical carriers of an organism s genetic information. Enzyme-catalyzed hydrolysis of nucleic acids yields nucleotides, the monomer units from which RNA and DNA are constructed. Further enzyme-catalyzed hydrolysis of the nucleotides yields nucleosides plus phosphate. Nucleosides, in turn, consist of a purine or pyrimidine base linked to Cl of an aldopentose sugar—ribose in RNA and 2-deoxyribose in DNA. The nucleotides are joined by phosphate links between the 5 phosphate of one nucleotide and the 3 hydroxyl on the sugar of another nucleotide. 

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

Fig. 2. The simple asymmetric carrier model for glucose transport. C denotes a sugar-binding site, which can exist in an outward-facing (Co) or an inward-facing (Ci) conformation. Dissociation constants for sugar binding are bja and ejf. Rate constants for carrier re-orientation are c, d, g, and h.

Because the rates of sugar binding to and dissociation from the transporter are very rapid compared to the rates of transporter re-orientation, the Michaelis constants for transport by the simple asymmetric carrier model are given by the following equations,... 

Using these equations, Lowe and Walmsley [48] have calculated the dissociation constants for sugar binding at the extracellular surface of the membrane (K s = b a in Fig. 2) and at the cytoplasmic surface (K. = elf = bid) x [dgich]) from the estimated rate constants for carrier re-orientation and the measured Michaelis constants. The dissociation constant for binding at the extracellular surface of the membrane, calculated in this way, is approximately lOmM and is largely unaffec-... 

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