Alcohol substrate inhibition

For hydrogenation to take place, the substrate usually needs to bind to the metal complex, although exceptions are known to this rule [25]. Substrate inhibition can occur in a number of ways, for example if more than one molecule of substrate binds to the metal complex. At low concentration this may be a minor species, whereas at high substrate concentration this may be the only species. One example of this is the hydrogenation of allyl alcohol using Wilkinson s catalyst. Here, the rate dependence on the substrate concentration went through a maximum at 1.2 mmol IT1. The authors propose that this is caused by formation of a complex containing two molecules of allyl alcohol (Scheme 44.1) [26],... 

Scheme 44.1 Substrate inhibition in the hydrogenation of allyl alcohol.

Horse Uver alcohol dehydrogenase Inhibited by aliphatic, cychc and aromatic oximes using ethanol as substrate 123... 

The first true latent inhibitor for ADH (i.e. an inhibitor produced by the catalytic act of the enzyme that is time dependent) was reported as being the substituted allyl alcohol, 3-ethylthioprop-2-en-l-ol.1428 This is a substrate for LADH, but after a short period the enzyme loses its activity, the rate of deactivation being proportional to the concentration of the alcohol. Latent inhibition was confirmed by showing directly that the aldehyde is an inhibitor. A mechanism is proposed in which the aldehyde formed reacts with an unspecified nucleophilic site on the enzyme with subsequent protonation.1429... 

In Eq. (3) [S]o and [S]f are starting and ending substrate concentrations. S approaches [S] when substrate consumption is minimal, and S is substituted for [S] to correct for excess substrate consumption. In these analyses, however, substrate inhibition can be a problem if the product has a similar affinity to the substrate. Fortunately, most P450 oxidations produce products that are less hydrophobic than the substrates, resulting in lower affinities to the enzymes. There are exceptions, including desaturation reactions that produce alkenes from alkanes (10) and carbonyl compounds from alcohols. These products have hydrophobicities that are similar or increased relative to their substrates. 

The kinetics of the new commercial process of hydroformylation of allyl alcohol was studied by Chaudhari in the temperature range from 60 to 80 °C [114]. The rate of reaction is first order in catalyst concentration and 1.5th order in hydrogen partial pressure. The dependence on p CO) does not differ from that observed in the hydroformylation of nonfunctionalized olefins. The reaction is retarded at higher substrate concentrations (> 1.25 mol/L). This substrate inhibition is not fully understood on the molecular level. The apparent activation energy for the oxo reaction of allyl alcohol was found to be 94 kJ/mol. 

Side note Methanol Poisoning. An interesting and imponant example competitive substrate inhibition is the enzyme alcohol dehydrogenase (AC in the presence of ethanol and methanol. If a person ingests methanol, Al will convert it to formaldehyde and then formate, which causes blindne Consequently, the treatment involves intravenously injecting ethanol (wh is metabolized at a slower rate than methanol at a controlled rate to tie ADH to slow the metabolism of methanol-to-formaldehyde-to-formate so l the kidneys have time to filter out the methanol which is then excreted in urine. With this treatment, blindness is avoided. For more on the met nol/ethanol competitive inhibition, see Problem P7 2Sc. 

Dalziel and Dickinson (34 ) found that this pathway contributed significantly to the kinetic parameters when cyclohexanol was used as a substrate. Isotope exchange studies 343) have confirmed the partly random mechanism for this substrate. Shore and Theorell (344) have studied the substrate inhibition for several aliphatic alcohols resulting from the formation of abortive E-NADH> alcohol complexes. 

When Rudolph and Fromm used thionicotinamide adenine dinucleotide (thio-DPN) as an alternate substrate for NAD+ and varied the concentration of ethanol with liver alcohol dehydrogenase [following the reaction at 342 nm, the isosbes-tic point for thio-DPN and reduced thio-DPN (thio-DPNH)], they saw what appeared to be concave upward reciprocal plots with partial substrate inhibition in the presence of thio-DPN (38). However, the asymptote intercepts appeared to decrease with increased thio-DPN concentration, which is not what the above equations predict for a case where a minimum is present in the curve. There must have been other interactions that caused the substrate inhibition by ethanol in the presence of thio-DPN. 

Polar solvents inhibit the reaction, presumably by interfering with the adsorption process as noted in the mechanism proposed for manganese dioxide oxidations. Oxidation of 1-heptanol to heptanal with Fetizon s reagent was quantitative when the solvent was 35% hexanes. When benzene was used as a solvent, the yield of heptanal dropped to 90% and was < 1% in ethyl acetate, methyl ethyl ketone, or acetonitrile. 69 Since the oxidation is a heterogeneous reaction, requiring adsorption of the alcohol substrate, as the surface area of the reagent increases (increased by precipitation on Celite), the rate of oxidation increases. An optimum ratio is reached beyond which increasing the silver carbonate/Celite ratio slows the oxidation. 69... 

When benzyl alcohol, 2,4-dinitrophenol, dioxane, etc. were added to the reaction system, the competitive inhibition was observed as inferred from the change of the Lineweaver-Burk plot (70). The fact that neutral molecules such as benzyl alcohol competitively inhibit the catalysis, indicates that the hydrophobic nature of the catalytic site makes a major contribution to substrate binding. 

Bousquet-Dubouch MP, Graber M, Sousa N et al. (2(X)1) Alcoholysis catalyzed by Candida antarc-tica lipase B in a gas/soUd system obeys a ping pong bi bi mechanism with competitive inhibition by the alcohol substrate and water. Biochim Biophys Acta 1550 90-99 Briggs GE, Haldane IBS (1925) A note on the kinetics of enzyme action. Biochem J 19 338-339 Buchholz K, Kasche V, Bomscheuer UT (2005) Biocatalysts and enzyme technology. Whey VCH, Weinhein, 448 pp... 

Due to the fact that the lipophilic aldehydic or ketonic products are often more tightly bound onto the hydrophobic active site of the enzyme than the more hydrophihc substrate alcohol, product inhibition is a common phenomenon in such reactions [759]. 

Ezeji TC, Qureshi N, Blaschek HP (2004b) Acetone butanol ethanol (ABE) production from concentrated substrate reduction in substrate inhibition by fed-batch technique and product inhibition by gas stripping. Appl Microbiol Biotechnol 63(6) 653-8 Ezeji T, Qureshi N, Blaschek H (2005) Industrially relevant fermentations. In Dtirre P (ed) Handbook on clostridia. Taylor Francis, p. 797-812 Ezeji T, Qureshi N, Blaschek H (2005) Process for continuous solvent production. Google Patents Ezeji T et al (2006) Butanol production from com. In Minteer SD (ed) Alcoholic fuels. CRC/Taylor Francis... 

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