Coenzyme bacterial synthesis

The mechanism for bacterial synthesis of PHA is not the simple dehydration reaction between alcohol and carboxyl groups. It is more complicated and involves the coenzyme A thioester derivative of the hydroxyalkanoic acid monomer (produced from the organic feedstock available to the bacteria) [Kamachi et al., 2001], Growth involves an acyl transfer reaction catalyzed by the enzyme PHA synthase (also called a polymerase) [Blei and Odian,... 

Sulfonamides are structural analogs of PABA that competitively inhibit bacterial synthesis of folic acid (see p. 371). Because purine synthesis requires THF as a coenzyme, the sulfa drugs slow down this pathway in bacteria. 

Dietary deficiency is relatively widespread, yet is apparently never fatal there is not even a clearly characteristic riboflavin deficiency disease. In addition to intestinal bacterial synthesis of the vitamin, there is very efficient conservation and reutilization of riboflavin in tissues. Flavin coenzymes are tightly enzyme bound, in some cases covalently, and control of tissue flavins is largely at the level of synthesis and catabolism of flavin-dependent enzymes. 

The first synthetic antibiotics were the sulfonamides (right). As analogues of p-ami-nobenzoic acid, these affect the synthesis of folic acid, an essential precursor of the coenzyme THF (see p. 108). Transport antibiotics (top center) have the properties of ion channels (see p. 222). When they are deposited in the plasma membrane, it leads to a loss of ions that damages the bacterial cells. 

The mode of action of the sulfonamides as antagonists of 4-aminobenzoic acid (PAB) is well documented, as is the effect of physicochemical properties of the sulfonamide molecule, e.g. pK, on potency (B-81MI10802). Sulfonamides compete with PAB in the biosynthesis of folic acid (44), a vital precursor for several coenzymes found in all living cells. Mammalian cells cannot synthesize folic acid (44), and rely on its uptake as an essential vitamin. However, bacteria depend on its synthesis from pteridine precursors, hence the selective toxicity of sulfonamides for bacterial cells. Sulfonamides may compete with PAB at an enzyme site during the assembly of folic acid (44) or they may deplete the pteridine supply of the cell by forming covalently-bonded species such as (45) or they may replace PAB as an enzyme substrate to generate coupled products such as (46) which are useless to the cell. 

Low-molecular-mass thiols such as coenzyme A and protein-bound thiol cofactors such as phospho-pantetheine are present in all cells. Their SH groups can also be oxidized to disulfides and it is of interest that in resting bacterial spores these compounds exist largely as disulfides or mixed disulfides. Upon germination of the spores special enzymes reduce the disulfides.136 Some proteins involved in control of protein synthesis contain SH groups that add covalently to C-6 atoms of a uracil ring in specific mRNA molecules. Control of their state of reduction may also be important.137... 

B Shell, CR Hutchinson. Enzymatic synthesis of a bacterial polyketide from acetyl and malonyl coenzyme A. Science 262 1535-1540, 1993. 

Fatty acid synthesis starts with acetyl-CoA, and the chain grows from the tail end so that carbon 1 and the alpha-carbon of the complete fatty acid are added last. The first reaction is the transfer of the acetyl group to a pantothenate group of acyl carrier protein (ACP), a region of the large mammalian FAS protein. (The acyl carrier protein is a small, independent peptide in bacterial FAS, hence the name.) The pantothenate group of ACP is the same as is found on Coenzyme A, so the transfer requires no energy input ... 

The active form of folate is the tetrahydro-derivative that is formed through reduction by dihydrofolate reductase. This enzymatic reaction (Figure 29.5) is inhibited by trimethoprim, leading to a decrease in the folate coenzymes for purine, pyrimidine, and amino acid synthesis. Bacterial reductase has a much stronger affinity for trimethoprim than does the mammalian enzyme, which accounts for the drug s selective toxicity. [Note Examples of other folate reductase inhibitors include pyrimethamine, which is used with sulfonamides in parasitic infections (see p. 353), and methotrexate, which is used in cancer chemotherapy (see p. 378).]... 

Tetrahydrofolic acid (THF) is a coenzyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and are required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation. Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing it-more precisely DHF-ffom precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

The Pfitzinger reaction has been used in the synthesis of methoxatine, a coenzyme of the bacterial enzyme alcohol dehydrogenase491,492 (Scheme 120) and of DuP 785, an anticancer agent493 (Scheme 121). 

Various aspects of bacterial nucleotides and nucleosides have been included in a number of recent reviews on such subjects as purine and pyrimidine synthesis, coenzymes, and carbohydrate polymers. ... 

PQQ is present as a noncovalently bound coenzyme in bacterial enzymes, and organisms that are incapable of its de novo synthesis can import it from the culture medium. It is synthesized by reaction between glutamate and tyrosine residues in a small (24 amino acid) peptide that is coded for by one of the bacterial genes known to be required for PQQ synthesis (Stites et af., 2000b). 

We have used the reaction extensively to prepare the indole moiety of several natural products. For example, the key step in the synthesis of the bacterial coenzyme methoxatin (36) is the formation of the indole (35) by intramolecular nitrene insertion from the azide (34), readily prepared from commercially available 4-aminosalicyclic acid. The third ring was annelated onto the indole (35) using conventional chemistry to give, after oxidation to the orrho-quinone, the natural product (36). 

After screening various microorganisms, scientists at Bristol-Myers Squibb selected a bacterial strain of Acinetobacter calcoaceticus SC 13876 to reduce a 3,5-dioxo ester 14 to the dihydroxy ester 15 (Scheme 10) [66], The diol 15 is a key intermediate in the synthesis of an 3-hydroxy-3-methylglutanyl coenzyme A (HMG-CoA) reductase inhibitor (16). In a one-liter batch reaction, a yield of 92% was obtained with an optical purity of 99%. 

Another enzyme for which X-ray diffraction studies have aided in an analysis of the mode of action is the enzyme dihydrofolate reductase. This catalyzes the reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate, an essential coenzyme used in the synthesis of thymidylate, inosinate, and methionine. The antitumor agent methotrexate is a powerful inhibitor of dihydrofolate reductase, causing, on binding, a cellular deficiency of thymidylate (the cause of its antitumor activity). The crystal structures of the enzyme from two bacterial sources—Escherichia coli and Lactobacillus casei—and from chicken liver have been studied (88-90). Both the E. coli and L casei enzymes have been studied as complexes with methotrexate bound at the active site, and, in the case of the . casei enzyme, the cofactor, NADPH, was also present. 

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