Hydration stereospecific

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Hydroboration-oxidation (Sections 6.11-6.13) This two-step sequence achieves hydration of alkenes in a stereospecific syn manner, with a regiose-lectivity opposite to Markovnikov s rule. An organoborane is formed by electrophilic addition of diborane to an alkene. Oxidation of the organoborane intermediate with hydrogen peroxide completes the process. Rearrangements do not occur. 

Fumarate is hydrated in a stereospecific reaction by fumarase to give L-malate (Figure 20.17). The reaction involves fraw5-addition of the elements of water across the double bond. Recall that aconitase carries out a similar reaction. 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Kinetic inhibitors for hydrate formation may also be effective in preventing scale deposition [1627]. This may be understood in terms of stereospecific and nonspecific mechanisms of scale inhibition. 

The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least-substituted carbon of a double bond results in the alcohol being formed with regiochemistry that is complementary to that observed by direct hydration or oxymercuration, that is, anti-Markovnikov. 

It was shown that microsomal epoxide hydrolase-catalyzed trans-addition of water to BaP 9,10-epoxide occurs stereospecifically at the C-9 position (15). Since BaP is metabolized essentially to an optically pure 9R,10R-dihydrodiol (13 and L5 Table I), the 9,10-epoxide formed in BaP metabolism must have 9S,10R absolute stereochemistry (Figure 1). Similarly, the 7,8-epoxide formed in BaP metabolism is hydrated specifically at the C-8 position to form the 7R,8R-dihydrodiol (14.21). Hence the enzymatically formed 7,8-epoxide intermediate has 7R,8S absolute stereochemistry (Figure 1). Although the 7R,8R-dihydrodiol is formed almost exclusively from BaP metabolism in rat liver microsomes (Table I) and in bovine bronchial explants (25). the 7S,8S-dihydrodiol is also formed from BaP metabolism in mouse skin epidermis in vivo (5). 

Boyd, D.R., McMordie, R.A S., Sharma, N.D., Dalton, H., Williams, P. and Jenkins, R.O., Stereospecific benzylic hydroxylation of bicyclic alkenes by Pseudomonas putida isolation of (+)-/ -l-hydroxy-l,2-dihydronaphthalene, an arene hydrate of naphthalene from metabolism of... 

C-3 as determined by mass spectral analysis.(55) In a mechanism involving ketone hydration prior to bindTrTg, incorporation in recovered inhibitor should be at least 50%, a value corresponding to that expected for a single cycle of nonstereospecific addition/nonstereospecific elimination of water to the ketone carbonyl. The actual results then indicate that addition-elimination is a highly stereospecific process and thus enzyme-catalyzed. 

According to the second mechanism,414 formation of the intermediate 2-acetamidoglycal (105) from the sugar nucleotide is an irreversible step of the process. This derivative then undergoes further stereospecific hydration, leading to the product 101. 

The final, critical oxidative spirocyclization of the 2,3-disubstituted indole to the spiro oxindole was effected by treatment of 124 with tert-butyl hypochlorite in pyridine to provide the labile 125 [Fig. (34)]. The Pinacol-type rearrangement was conducted by treating compound 125 with p-toluenesulfonic acid in THF/water. It is assumed that the chlorination of 124 proceeds from the least hindered face of the indole, to give the a-chloroindolene 125. The hydration of the imine functionality must also occur from the same a-face that is syn to the relatively large chlorine atom furnishing the syn-chlorohydrin 126, that subsequently rearranges stereospecifically to the desired spiro oxindole 127. 

Yang, W., L. Dostal, and J. P. N. Rosazza, Stereospecificity of microbial hydrations of oleic acid to 10-hydroxystearic acid , Appl. Environ. Microbiol., 59, 281-284 (1993). 

This enzyme is highly stereospecific it catalyzes hydration of the trans double bond of fumarate but not the cis double bond of maleate (the cis isomer of fumarate). In the reverse direction (from L-malate to fumarate), fumarase is equally stereospecific D-malate is not a substrate. 

Only a few examples are known in which an enzyme induces addition to a double bond that is not conjugated with a carbonyl or carboxyl group. Pseudomonads have been observed to catalyze stereospecific hydration of oleic acid to D-10-hydroxystearate.96 The addition is anti and the proton enters from the re face. 

Pteridine is changed by, the enzyme adenosine deaminase to the levorotatory form of the hydrate, 3,4-dihydro-4-hydroxypteridine. When this reaction was approached from the other direction, the same enzyme dehydrated this enantiomer most rapidly, leaving a net positive optical rotation at equilibrium.65 Specimens of the enzyme from both mammalian and fungal sources, although of very different molecular weight, were found to catalyze the stereospecific hydration of pteridine also pteridine inhibited the deamination of adenosine by both enzyme specimens. Assuming that the first step in the deamination of adenosine is the formation of a tetrahedral hydrated intermediate, it was argued that both the hydration of pteridine and the deamination of adenosine were parallel phenomena.65... 

Figure 10-9 Representation of the course of enzyme-induced hydration of fumaric acid (trans-butenedioic acid) to give L-malic acid (L-2-hydroxy-butanedioic acid). If the enzyme complexes with either—C02H (carboxyl) group of fumaric acid, and then adds OH from its right hand and H from its left, the proper stereoisomer (l) is produced by antarafacial addition to the double bond. At least three particular points of contact must occur between enzyme and substrate to provide the observed stereospecificity of the addition. Thus, if the enzyme functions equally well with the alkenic hydrogen or the carboxyl toward its mouth (as shown in the drawing) the reaction still will give antarafacial addition, but o,L-malic acid will be the product.

The reaction is remarkable for a number of reasons. It is readily reversible and is catalyzed by an enzyme (fumarase) at nearly neutral conditions (pH s 7). Without the enzyme, no hydration occurs under these conditions. Also, the enzymatic hydration is a completely stereospecific antarafacial addition and creates L-malic acid. The enzyme operates on fumaric acid in such a way that the proton adds on one side and the hydroxyl group adds on the other side of the double bond of fumaric acid. This is shown schematically in Figure 10-9. 

However, various attempts to introduce stereospecifically the a-hydroxyl group at the desired position of the double bond by hydroboration were unsuccessful. Eventually hydration of the double bond was accomplished by mercuration-reduction protocol, which although occurring both with high regio and stereoselectivity furnished only the p hydroxy compound 339. The conversion of the latter with formaldehyde into ( )-epielwesine (335) constituted in a formal sense, the synthesis of ( )-elwesine (320) as well, since Sanchez et al 88 had shown that the inversion of the hydroxyl group in 335 could be accomplished with diethylazodicarboxylate, triphenylphosphine and formic acid. 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

---