Side chains proposal

Carbonic anhydrase is an enzyme that catalyzes the conversion of carhon dioxide into bicarbonate ion (Section 2.2). It is a metalloenzyme, with Zn coordinated at the active site by three histidine side chains. Propose a mechanism for the reaction. 

Based on the structure of hog submaxillary mucin oligosaccharide side chains proposed by Baig and Aminoff (1972), another sialyltransfer-ase, responsible for the addition of sialic acid linked to j8-galactosyl residues, remains to be isolated from hog submaxillary tissues (see Figure 11). 

FIGURE 27 19 Proposed mechanism of hydrolysis of a peptide catalyzed by carboxypeptidase A The peptide is bound at the active site by an ionic bond between its C terminal ammo acid and the positively charged side chain of arginine 145 Coordination of Zn to oxygen makes the carbon of the carbonyl group more positive and increases the rate of nucleophilic attack by water... 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

The mold metabolite mycelianamide, isolated from the mycelium of strains of Penicillium gritieofulvum Dierckx, was first investigated by Oxford and Raistrick. The reinterpretation and extension of this work by Birch et and the revision by Bates et of the structure first proposed for the terpenoid side-chain, have led to the formulation of mycelianamide as 11. This structure has been confirmed by further degradations and by a synthesis of racemic deoxymyceli-anamide by Gallina and co-workers. The ready decomposition of the heterocyclic ring by either acid or alkali is discussed later (Section... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

By structural complementarity, dicationic l,4-diazabicyclo[2.2.2]octane (VII) provides an appropriate recognition site for phosphate ions and two stearyl side chains attached to the amines add lipophilic properties 59,60). Such a carrier model can selectively extract nucleotides from aqueous solution to chloroform solution via lipophilic salt formation. The order of nucleotide affinity is ATP > ADP > AMP. The selectivity ratios were 45 for ADP/AMP and 7500 for ATP/AMP at pH 3. The relative transport rate was ATP > ADP > AMP. The ratios were 60 for ATP/AMP and 51 for ADP/AMP. The modes of interaction of ADP and ATP are proposed to be as shown in Fig. 6. 

The data in Table 191 show a large difference in entropy of activation for reaction of the 4-methoxy compound in the three acids. The difference in the data for perchloric and sulphuric acids was again thought to arise from a significant amount of reaction in the latter occurring via bisulphate anion. This anion was proposed as being insufficiently electrophilic to react except by coordination with the side chain, as in scheme (2S0)... 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

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