Carbon-substrate inhibition

The kinetics used here consider carbon-substrate inhibition and oxygen limitation. Thus,... 

Fucosyltransferases (glycosyltransferases) promote the transfer of fucose from GDP-fucose onto a saccharidic acceptor. This transfer occurs with inversion of the configuration of the anomeric carbon. The inhibition has been studied with fluorinated substrates. GDP-2-fluoro-2-deoxyfucoses (GDP-2F-fucose) and GDP-6-fluoro-6-deoxyfucoses (GDP-6F-fucose) are competitive inhibitors. The values are close to or less than that of (Figure 1.29)P The very close values of GDP-2F-fucose... 

The importance of the interaction of organic compounds with calcium carbonate surfaces has long been recognized. It has been demonstrated that aragonite precipitation is inhibited by uncharacterized dissolved organic matter (Chave and Suess, 1967), and that humic and fulvic acids, and certain aromatic carboxylic acids, inhibit seeded aragonite precipitation from seawater (Berner et al., 1978). The selective adsorption of amino acids on carbonate substrates has received considerable attention. A preferential adsorption of aspartic acid has been shown from humic and fulvic acids and proteinaceous matter (Carter and Mitterer, 1978 Carter, 1978 Mitterer, 1971). 

The most common desaturase in most organisms, including insects, is stearoyl Co A desaturase, which introduces a double bond in the 9-10 position of long-chain fatty acids (2JL). Similarities between this enzyme and the All desaturase from cabbage looper include location in the microsomal fraction, lack of sensitivity to carbon monoxide, inhibition by cyanide, use of a reduced nicotine-adenine nucleotide cofactor as an electron source and use of 16 and 18 carbon acids as preferred substrates. 

O2 is typically a substrate that in high concentrations leads to substrate inhibition, but a high concentration of the carbon source can also be inhibiting (e.g., in bioremediation of toxic waste a high concentration of the organic substrate can well lead to severe inhibition or death of the microorganism). 

The work also revealed that unlike in conventional systems, there was no substrate inhibition and no critical catalyst concentration loading, which is often brought about by the formation of an inactive catalytic dimer species. The novel catalyst or the presence of CO2 was suggested to be the cause of this effect. Carbon monoxide was found to have a strong effect on the selectivity of linear vs. branched product in the supercritical environment. 

It could be shown, moreover, (Vallee and Neurath, 1955) that five times recrystallized carboxypeptidase was completely inhibited by metal chelating agents, such as 8-OHQ-5SA and 1-10 phenthroline at concentrations of 10 W,Q ,Q Dat concentrations of 10" Af, and some 30% byEDTA at 10 M. These are all known to form complexes with zinc in simple systems. In these experiments, the buffered enzyme solutions were incubated with the chelating agent at pH 7.5, 4°C., for 1 hour prior to the addition of the substrate. Inhibition did not occur when these chelating agents were first incubated with an equimolar amount of zinc, cupric, or ferrous ions. Sodium diethyldithiocarbamate, zincon, sulfanilamide, and diamox, the latter two employed because of their effect on carbonic anhydrase, had little, if any, effect on carboxypeptidase activity. DPN, nicotinamide, and A-methylnicotinamide, examined because of their effect on the ADH sys-... 

To be more specific, we refer to some known systems obeying the nonlinear kinetic laws mentioned above one of them involves a wide variety of the reactions of catalytic combustion of carbon oxide and hydrocarbons described by models with substrate inhibition (see, for example, reference 25) another one is connected with ammonium oxidation on platinum characterized by a chain-branched multiplication of active centers. ... 

It has been reported that use of a suitable co-solvent increases the concentration of the olefin in water (catalyst) while retaining the biphasic nature of the system. It has been shown that using co-solvents like ethanol, acetonitrile, methanol, ethylene glycol, and acetone, the rate can be enhanced by several times [27, 28], However, in some cases, a lower selectivity is obtained due to interaction of the co-solvent with products (e.g., formation of acetals by the reaction of ethanol and aldehyde). The hydroformylation of 1-octene with dinuclear [Rh2(/t-SR)2(CO)2(TPPTS)2] and HRh(CO)(TPPTS)3 complex catalysts has been investigated by Monteil etal. [27], which showed that ethanol was the best co-solvent. Purwanto and Delmas [28] have reported the kinetics of hydroformylation of 1-octene using [Rh(cod)Cl]2-TPPTS catalyst in the presence of ethanol as a co-solvent in the temperature range 333-353 K. First-order dependence was observed for the effect of the concentration of catalyst and of 1-octene. The effect of partial pressure of hydrogen indicates a fractional order (0.6-0.7) and substrate inhibition was observed with partial pressure of carbon monoxide. A rate eqution was proposed (Eq. 2). 

The effect of reaction parameters, such as the concentrations of catalyst and olefin and the partial pressures of CO and hydrogen, on the rate of reaction has been studied at 373 K [19]. The rate varies linarly with catalyst concentration, olefin concentration, and partial pressure of hydrogen. Typical substrate-inhibited kinetics was observed with the partial pressure of carbon monoxide. Further, a rate equation to predict the observed rate data has been proposed (Eq. 5). 

It is important to note that the kinetic trend were completely opposite for cases with and without catalyst binding ligand for carbon monoxide. Since, under conditions of interfacial catalysis, a higher CO concentration is accessible to the catalytic species, substrate inhibition is observed. 

All four carboxylases use bicarbonate as their one-carbon substrate and, in all, the biotin is covalently linked by an amide bond between the carboxyl of biotin and an epsilon amino group of a lysyl residue in the holocarboxylase synthase (= biotin ligase) that catalyzes the formation of the covalent bond. Biotinylation of histones is involved in regulation of gene transcription and may also play a role in packaging of deoxyribonucleic acid (DNA). Biotin has also been found to inhibit the generation of reactive oxygen species (ROS) by neutrophils in vitro. 

We designate this reaction electron activation and the site(s) involved as the electron-activating site. The other type involves reduction of added reducible substrates and the additional site involved is referred to as the substrate-complexing site. The two sites are distinguished by inhibition experiments. Carbon monoxide inhibits reactions at the substrate-complexing site, but not at the electron-activating site. 

The kinetics of hydroformylation of 1-octene using [Rh(COD)Cl]2 as a catalyst precursor with TPPTS as a water-soluble ligand and ethanol as a co-solvent was further studied by Deshpande et al. [15] at different pH values. The rate increased by two- to fivefold when the pH increased from 7 to 10, while the dependence of the rate was found to be linear with olefin and hydrogen concentrations at both pH values. The rate of hydroformylation was foimd to be inhibited at higher catalyst concentrations at pH 7, in contrast to linear dependence at pH 10 (Figure 5). The effect of the concentration of carbon monoxide was linear at pH 7, in contrast to the usual negative-order dependence. At pH 10, substrate-inhibited kinetics was observed with respect to CO (Figure 6). 

Substrates arising from agricultural raw materials typically consist of several macromolecules. A structural macromolecule may provide an inert matrix within which the carbon source, such as starch molecules or soluble sugars, are located, or the structural macromolecule itself may be the carbon source. The complexity of such substrates can lead to phenomena such as nutrient limitation, substrate inhibition, catabolite repression, and complex patterns of induction and repression of extracellular enzymes. This can have consequences for growth and sporulation. 

Some attention has been given to C N ratios. However, note that overall C N ratios do not necessarily reflect the relative availabilities of the carbon and nitrogen sources because the nutrients present are not necessarily equally accessible to the organism. Therefore optimal C N ratios in SSF can potentially be quite different from those found in SLF. Also, even though the substrate may contain high levels of carbon, the provision of sufficient levels of the nitrogen source at the start of the fermentation to enable complete utiHzation of the carbon source can potentially cause substrate inhibition [78]. 

---