Keto reduction

When the catalytic reaction of 6-hydroxymellein synthase is carried out in the absence of NADPH or with monomeric enzyme, keto-reduction of the carbonyl group of the triketomethylene chain does not take place, and the synthase liberates triacetic acid lactone instead of 6-hydroxymellein [64, 71]. However, the efficiencies of product formation are markedly lower than for the normal reaction. Two mechanisms could account for the low efficiency of triacetic acid lactone formation observed in the monomeric and the NADPH-depleted dimeric forms of 6-hydroxymellein synthase. These are 1) Reduced affinity of the cosubstrates acetyl-CoA and/or malonyl-CoA for the enzyme protein with the incomplete reaction centers 2) Reduced rate of reaction of acyl-CoA condensation and/or product liberation. To examine these possibilities, kinetic parameters of the two triacetic acid lactone-forming reactions were compared with those of the normal reaction which produces 6-hydroxymellein. The Km value of 6-hydroxymellein synthase for acetyl-CoA in the normal reaction was estimated to be 22 pM, while in both the NADPH-depleted dimer and the monomer reactions the affinity of 6-hydroxymellein synthase protein for acetyl-CoA was markedly lower at 284 and 318 pM respectively. By contrast the Km values for malonyl-CoA in the normal and the two abnormal reactions were essentially the same (40 - 43 pM), indicating that the affinity of 6-hydroxymellein... 

Like the related fatty acid synthases (FASs), polyketide synthases (PKSs) are multifunctional enzymes that catalyze the decarboxylative (Claisen) condensation of simple carboxylic acids, activated as their coenzyme A (CoA) thioesters. While FASs typically use acetyl-CoA as the starter unit and malonyl-CoA as the extender unit, PKSs often employ acetyl- or propionyl-CoA to initiate biosynthesis, and malonyl-, methylmalonyl-, and occasionally ethylmalonyl-CoA or pro-pylmalonyl-CoA as a source of chain-extension units. After each condensation, FASs catalyze the full reduction of the P-ketothioester to a methylene by way of ketoreduction, dehydration, and enoyl reduction (Fig. 3). In contrast, PKSs shortcut the FAS pathway in one of two ways (Fig. 4). The aromatic PKSs (Fig. 4a) leave the P-keto groups substantially intact to produce aromatic products, while the modular PKSs (Fig. 4b) catalyze a variable extent of reduction to yield the so-called complex polyketides. In the latter case, reduction may not occur, or there may be formation of a P-hydroxy, double-bond, or fully saturated methylene additionally, the outcome may vary between different cycles of chain extension (Fig. 4b). This inherent variability in keto reduction, the greater variety of... 

Figure 11.20 Screening of the polyamine amide catalyst hbrary L14 using the keto reduction of 11.22.

Disposition in the Body. Readily absorbed after oral administration. Metabolised by A-dealkylation, reduction, deamination, and A-hydroxylation primarily to active metabolites keto-reduction is stereoselective resulting in the formation of threo-hydroxylated metabolites glucuronide formation also occurs along with the formation of hippuric and mandelic acids. About 80 to 90% of a dose is excreted in the urine the amount excreted in the urine is reduced when the urine is alkaline of the urinary excreted material, A-ethylaminopropiophenone, norephedrine (phenylpropanolamine), and hippuric acid are the main metabolites together with small amounts of unchanged drug, amino-propiophenone, A-diethylnorephedrine, and A-ethylnor-ephedrine. 

Methylmalonyl-CoA DEBS ATs P-keto reduction DEBS KR5 RAPS KR2"... 

Figure 31 Bacillaene biosynthesis, (a) Structure of dihydrobacillaene and bacillaene. PksJ oxidizes the C14 -C15 bond after dihydrobacillaene has been synthesized. Also, note the a-hydroxyacyl N-cap. This particular N-capping has been reported very rarely. (b)a- and /3-Ketoreduction of a-KICto a-HIC. The KR domain of the first PKS module in PksJ is capable of reducing both the a-KIC amide and the /3-ketone in an NAD(P)H-dependent fashion. The order in which these two reductions occur is unknown. Ultimately, keto-reduction is followed by dehydration and enoyl reduction, (c) Theoretical structure of PPant ejection ions used to analyze PksJ ketoreduction. Right PPant ejection ion resulting from IRMPD of Acac-S-PksJ(T3-T4) incubated with PksJ. Mass shift of +2.017 Da corresponds with reduction of the /3-ketone. Left PPant ejection ion resulting from IRMPD of a-KIC-GABA-S-PksJ(T3-T4) incubated with PksJ. Shift of +2.015 Da is observed in PPant ejection ions.

Fig. 3. The assembly of fatty acids, polyketides and reduced polyketides. The reduced polyketide intermediate would be formed from an acetate starter by five successive condensation cycles. The first two cycles are condensations and are followed by condensation-ketoreduction, condensation-ketoreduction-elimination, and finally a full condensation-keto-reduction-elimination-enoyl reduction cycle. Thus the overall reaction sequence is A, A, AB, ABC,ABCD...

Neither 21-hydroxylation, ring A or 20-keto reduction is inhibited by Acutely hypoxed rats secrete more blue tetrazolium reducing (BT+) steroids following M suggesting a direct stimulation of the adrenal 3. However, rats secrete a BT- steroid, 18-OH DOC, and thus inhibition of 11- and 18-hydroxylations by M could result in an amount of BT+ DOC equal to B +18-0H DOC. While total BT+ material would increase, the total amount of steroid could be the same and a direct effect on the adrenal need not be considered. 

Benzphetamine is a methamphetamine whose tertiary amine also carries a benzyl group. The likelihood of metabolic N-debenzylation yielding methamphetamine would, of course, explain its effects. Diethylpropion (No. 14) is an interesting compound because of its particularly involved metabolic degradation, which is initiated by stepwise N-deethylation and keto reduction to a P-OH. These active metabolites, presumably with the parent compound, explain both anorexiant and CNS effects, as well as the reduced level of the latter when compared with amphetamine. Subsequent oxidations all the way to benzoic, hydrox-ybenzoic, and mandelic acids all lead to inactivation. 

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