Enzymes unsaturated carbonyl compounds

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

The Michael reaction involves conjugate addition of a nucleophile onto an a,P-unsaturated carbonyl compound, or similar system. Such reactions take place in nature as well, and some can be potentially dangerous to us. For example, the a,P-unsaturated ester ethyl acrylate is a cancer suspect agent. This electrophile can react with biological nucleophiles and, in so doing, bind irreversibly to the nucleophile, rendering it unable to carry out its normal functions. A particularly important enzyme that can act as a nucleophile is DNA polymerase, which is responsible for the synthesis of strands of DNA, especially as part of a DNA repair mechanism (see Section 14.2.2). The nucleophilic centre is a thiol grouping, and this may react with ethyl acrylate as shown. 

A wide variety of structures have been found to act as sources of the electrophilic groups of suicide inhibitors (Figure 7.5). These structures will only give rise to an electrophilic group if the compound containing the structure can act as a substrate for the enzyme. They often take the form of a,p unsaturated carbonyl compounds and imines formed by the reverse of a Michael addition at the active site of the enzyme. 

The od.p unsaturated carbonyl compounds and imines formed in this manner react by a type of Michael addition with nucleophilic groups (Nu), such as the OH of serine residues, the SH of cysteine residues and the w-NH2 of lysine residues, frequently found at the active sites of enzymes. 

There are several published examples of irreversible inhibition of protein kinases, typically involving a Michael addition reaction where there is a nucleophilic addition of a carbanion (such as an enzyme cysteine thiolate in the purine site) to an unsaturated carbonyl compound (Figure 4.4). Compounds working in this way include the EGFR-TK inhibitor, WZ4002 (compound 4.9).17... 

Several flavoproteins catalyze the oxidation or reduction of carbon-carbon bonds. Often, the substrates for these enzymes are ct,/3-unsaturated carbonyl compounds. The mechanism used to catalyze these types of reactions involves hydride transfers to or from N 5 of the flavin. In addition, an active site residue is involved in the acid/base chemistry needed for the reaction to occur. Several of these enzymes are involved in cellular energy metabolism. 

Section 18.15 Enzyme-Catalyzed Additions to a,/3-Unsaturated Carbonyl Compounds 773... 

Boy land, E. and Chasseaud, L.F., Enzymes catalysing conjugations of glutathione with alpha-beta-unsaturated carbonyl compounds, B/ocIiera. J., 109,651,1968. 

Reduction of ketones and a,/7-unsaturated carbonyl compounds using enzymes... 

A novel ERED from Shewanella oneidensis (SYE-4) is a potent recent addition to the ERED family. This enzyme reduces both cydic and linear unsaturated carbonyl compounds as well as substituted maleimides [105]. In particular, the fast and selective transformation of 2-methyl maleates 56 (99% conversion after 6 h, 98-99% ee) by SYE-4 may provide a well-defined alternative to the undisclosed ERED used in the pilot-scale production (70 g, 35 g/L) of dimethyl (R)-2-methylmaleate 57 employed by Almac (Scheme9.19) [106]. The Almac group further combined this redox enzyme with a hydrolase in a one-pot reaction. 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

The functions of flavoprotein enzymes are numerous and diversified.151-1533 A few of them are shown in Table 15-2 and are classified there as follows (A) oxidation of hemiacetals to lactones, (B) oxidation of alcohols to aldehydes or ketones, (C) oxidation of amines to imines, (D) oxidation of carbonyl compounds or carboxylic acids to a,(3-unsaturated compounds,... 

When a 2 -Cl or -F analog of UDP was used in place of the substrate an irreversible side reaction occurred by which Cl or F, inorganic pyrophosphate, and uracil were released 349 When one of these enzyme-activated inhibitors containing 3H in the 3 position was tested, the tritium was shifted to the 2 position with loss of Cl and formation of a reactive 3 -carbonyl compound (Eq. 16-24) that can undergo P elimination at each end to give an unsaturated ketone which inactivates the enzyme. This suggested that the Fe-tyrosyl radical abstracts an electron (through a... 

Introduction of a double bond. The /3-oxidation pathway begins when fatty acid forms a thiol ester with coenzyme A to give a fatty acyl CoA. Two hydrogen atoms are then removed from carbons 2 and 3 by an acyl CoA dehydrogenase enzyme to yield an ,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently in biochemical pathways and is usually carried out by the coenz5nne flavin adenine dinucleotide (FAJ ). Reduced FADH is the by-product. 

Cytochrome P450 enzymes catalyze dehydrogenation as well as oxygenation reactions, including the oxidation of saturated to unsaturated hydrocarbons, alcohols to carbonyl compounds, and amines to imines or other unsaturated products. The most extensively investigated of these reactions in terms of mechanism is the desaturation of valproic acid to 2-/i-propyl-4-pentenoic... 

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