Acceptor analog

Mass spectrometry enables the type of direct analyses described, but it does have its limitations. Online operation forces detection at infusion concentrations, in salty buffer and under complex mixture conditions. General ion suppression results from the buffer and mixture components, and mixture complexity can tax the resolution of even the best mass spectrometers. Increasing compound concentration is not the answer, as this leads to problems of solubility and increased compound consumption. We have found that the online method can work successfully for up to 100 compounds per analysis, but the false negative rate becomes appreciable [21]. As an alternative for ligand discovery purposes, we have developed a FAC-LC/MS system in which FAC effluent is sampled and analyzed by LC/MS [19]. This system offers the ability to concentrate mixture components and introduces another dimension to the data in order to tolerate more complex mixtures (Fig. 6.9). Using this system, we have screened approximately 1000 modified trisaccharide acceptor analogs targeting immobilized N-... 

It may also be reduced microbiologically to H28, in which it serves as an electron acceptor analogous to sulfate (e.g.. Equation (1)) ... 

Some sialyltransferases recognize modified CMP-Neu5Ac such as those in which the hydroxyl group at C-9 is replaced with an amido, fluoro, azido, acetamido, or benzamido group [60-63]. Acceptor analogs in which the aceta-mido function is replaced by an azide, phthalimide, carbamate, or pivaloyl functionality are also accepted by the enzyme [64]. 

Group 9 boratobenzene sandwich compounds have also been used as acceptors. Analogs of 45 (Figure 6) where the ( 7 -C6H6)Ru fragment is replaced by the isoelectronic Cp Rh or Cp Ir fragments have been studied using HRS... 

The authors suggest that in addition to Castle s intrinsic factor there may be an intramural intestinal B12 acceptor (analogous to apoferritin in iron absorption) which may be responsible for the partial mucosal block to Bi2 absorption in the intestine of normal human beings. A similar mechanism may be responsible for limiting the amount of B12 which can be absorbed even with added intrinsic factor in patients with pernicious anemia (see page 163). 

Combination of this structural motif with an acceptor analog led to somewhat more potent, yet stiU cmisiderably weak, inhibitors. Thus, for compound 102 IC50 1.5 mM could be determined (Fig. 16) [58, 59]. 

O. Hindsgaul, K.J. Kaur, G. Srivastava, M. Blaszczyk-Thurin, S.C. Crawley, L.D. Heerze, M.M. Palcic, Evaluation of deoxygenated oligosaccharide acceptor analogs as specific inhibitors of glycosyltransferases, J Biol Chem, 1991, 266, 17858-17862. 

S.H. Khan, S.C. Crawley, O. Kanie, O. Hindsgaul, A trisaccharide acceptor analog for N-acetylglucosaminyltransferase V which binds to the enzyme but sterically precludes the transfer reaction, J Biol Chem, 1993,268, 2468-2473. 

Scheme 4. Transition-state and acceptor analogs as inhibitors of pi,4-galactosyltransferase.

The acceptor analog 19, where the reactive 2-OH of the Gal unit was deoxy-genated, was found to be a competitive inhibitor [23], suggesting that the OH undergoing fucosylation is not essential for binding to al,2-FucT II. Compound 19 has a K[ value of 0.80 mM which is similar to the Km of the corresponding acceptor (0.20 mM). 

Acceptor analogs of LacNAc with OH-6 modified were also studied [82, 83]. The analogs showed mixed inhibition with Ki values in the mM range. The 6 -deoxy analog had a K value of 0.76 mM, similar to the K a value of the corresponding acceptor (0.90 mM). 

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