Reductions, biosynthetic

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

Vnother pathway of glucose catabolism, the pentose phosphate pathway, is the primary source of N/ E)PH, the reduced coenzyme essential to most reductive biosynthetic processes. For example, N/VDPH is crucial to the biosynthesis of... 

The coenzyme NADP+ differs from NAD+ only by the presence of a phosphate group (-P04=)on one of the ribose units (Figure 13.4). This seemingly small change in structure allows NADP+ to interact with NADP+-specific enzymes that have unique roles in the cell. For example, the steady-state ratio of NADP+/NADPH in the cytosol of hepato-cytes is approximately 0.1, which favors the use of NADPH in reductive biosynthetic reactions. This contrasts with the high ratio of NAD7NADH (approximately 1000 in the cytosol of hepatocytes), which favors an oxidative role for NAD+. This section summarizes some important NADP+ or NADPH-specific functions. 

Answer Within organelles, reaction intermediates and enzymes can be maintained at different levels from those in the cytosol and in other organelles. For example, the ATP/ADP ratio is lower in mitochondria than in the cytosol because the role of adenine nucleotides in the mitochondrial matrix is to accept a phosphoryl group, whereas the role in the cytosol is to donate a phosphoryl group. Similarly, different NADH/NAD+ and NADPH/NADP+ ratios reflect the reductive (biosynthetic) functions of the cytosol and the oxidative (catabolic) functions of the mitochondrial matrix. By segregating reaction sequences that share intermediates, the cell can regulate catabolic and anabolic processes separately. 

The principle products of this pathway are R-5-P and NADPH. Under reductive biosynthetic conditions where R-5-P is not needed the Pentose-P pathway can be used to completely oxidize G-6-P to 6 carbon dioxide molecules with the concomitant production of 12 NADPH s. Note also that when R-5-P is needed and NADPH is not needed for reductive bios)mthesis it can be made from F-6-P and Ga-3-P. 

The answer is a. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121—138. Wilson, pp 287-320.) The major contributor of electrons in reductive biosynthetic reactions is nicotinamide adenine dinucleotide phosphate (NADPH -I- H ), which is derived by reduction of NAD. NAD is formed from the vitamin niacin (also called nicotinate). Niacin can be formed from tryptophan in humans. In the synthesis of NAD, niacin reacts with 5-phosphoribosyl-l-pyrophosphate to form nicotinate ribonucleotide. Then, AMP is transferred from ATP to nicotinate ribonucleotide. Finally, the amide group of glutamate is transferred to the niacin carboxyl group to form the final product, NAD. NADP is derived from NAD by phosphorylation of the 2 -hydroxyl group of the adenine ribose moiety. The reduction of NADP to NADPH -I- H occurs primarily through the hexose monophosphate shunt. 

NADPH is usually involved in reductive (biosynthetic) reactions and serves as a source of electrons. In contrast, NAD is involved in oxidative reactions and serves as a sink for electrons. In cells and tissues, the ratio of NADPH/NADP tends to be 1 10 and 1 1000. NADPH is considered the primary source of reducing equivalents for GSH. 

Since ivermectin (= 22,23-dihydroavermectin B ) is obtained by catalytic reduction of avermectin B, the same procedure using tritium gas convenientiy affords tritiated ivermectin (22,23- [JT]-22,23-dihydroavermectin B ). The preparation of a tritiated derivative containing a 22,23-double bond starts with the readily available 5-ketone, which is reduced with [JT]-sodium borohydride stereospecificaHy to a 5- [JT]-derivative (40). Carbon-14 labeled avermectins can be obtained by a biosynthetic process using sodium (l- C)propionate as labeled precursor (48). 

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

Cells require a constant supply of N/ X)PH for reductive reactions vital to biosynthetic purposes. Much of this requirement is met by a glucose-based metabolic sequence variously called the pentose phosphate pathway, the hexose monophosphate shunt, or the phosphogluconate pathway. In addition to providing N/VDPH for biosynthetic processes, this pathway produces ribos 5-phosphate, which is essential for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. 

Although the biosynthetic cascade hypothesis predicts the co-occurrence of endiandric acids D (4) and A (1) in nature, the former compound was not isolated until after its total synthesis was completed in the laboratory (see Scheme 6). Our journey to endiandric acid D (4) commences with the desilylation of key intermediate 22 to give alcohol 31 in 95% yield. The endo side chain is then converted to a methyl ester by hydrolysis of the nitrile to the corresponding acid with basic hydrogen peroxide, followed by esterification with diazomethane to afford intermediate 32 in 92% overall yield. The exo side chain is then constructed by sequential bromination, cyanide displacement, ester hydrolysis (33), reduction, and olefination (4) in a straight-... 

Finally, the necessity arose for the synthesis of pentulose 21, labeled with, 3C on the central carbons, C-2 and C-3, for an independent biosynthetic study, which is reported in Section III.5.27 The doubly labeled ester 34 (Scheme 14) is readily available by a Wittig- Homer condensation of benzyloxyacetaldehyde with commercially available triethylphosphono-(l,2-l3C2)acetate. Chirality was introduced by the reduction of ester 34 to the allylic alcohol, which produced the chiral epoxide 35 by the Sharpless epoxidation procedure. This was converted into the tetrose 36, and thence, into the protected pentulose 37 by the usual sequence of Grignard reaction and oxidation. 

As noted, the alkaloid yield from the Beocin plants was low, which the authors suggested might be caused by the poor soil in which the plants were growing (Popovic et ah, 1992). One could ask whether the soil conditions to which they refer might be influential in the overall alkaloid biosynthetic processes in this species. It would be of interest to see experimental studies aimed at determining the effect of soil components on these processes. In the present case, it may be a lack of, or reduction in the activity of, the oxidase(s) necessary for the dimerization process (required to form the bibenzyldihydroisoquinolines) to occur. It is also possible that the lack of dimeric alkaloids may simply reflect a concentration effect caused by the edaphic conditions. These questions should be accessible to experiment. 

The activity of PK and NRPSs is often precluded and/or followed by actions upon the natural products by modifying enzymes. There exists a first level of diversity in which the monomers for respective synthases must be created. For instance, in the case of many NRPs, noncanonical amino acids must be biosynthesized by a series of enzymes found within the biosynthetic gene cluster in order for the peptides to be available for elongation by the NRPS. A second level of molecular diversity comes into play via post-synthase modification. Examples of these activities include macrocyclization, heterocyclization, aromatization, methylation, oxidation, reduction, halogenation, and glycosylation. Finally, a third level of diversity can occur in which molecules from disparate secondary metabolic pathways may interact, such as the modification of a natural product by an isoprenoid oligomer. Here, we will cover only a small subsection of... 

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