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Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction.

ABSTRACT: Transketolase catalyzes asymmetric C-C bond formation of two highly polar compounds. Over the last 30?years, the reaction has unanimously been described in literature as irreversible because of the concomitant release of CO2 if using lithium hydroxypyruvate (LiHPA) as a substrate. Following the reaction over a longer period of time however, we have now found it to be initially kinetically controlled. Contrary to previous suggestions, for the non-natural conversion of synthetically more interesting apolar substrates, the complete change of active-site polarity is therefore not necessary. From docking studies it was revealed that water and hydrogen-bond networks are essential for substrate binding, thus allowing aliphatic aldehydes to be converted in the charged active site of transketolase.

SUBMITTER: Marsden SR

PROVIDER: S-EPMC5573996 | biostudies-literature | 2017 May

REPOSITORIES: biostudies-literature

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Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction.

Marsden Stefan R SR Gjonaj Lorina L Eustace Stephen J SJ Hanefeld Ulf U

ChemCatChem 20170413 10

Transketolase catalyzes asymmetric C-C bond formation of two highly polar compounds. Over the last 30 years, the reaction has unanimously been described in literature as irreversible because of the concomitant release of CO<sub>2</sub> if using lithium hydroxypyruvate (LiHPA) as a substrate. Following the reaction over a longer period of time however, we have now found it to be initially kinetically controlled. Contrary to previous suggestions, for the non-natural conversion of synthetically mor ...[more]

PMID: 28919932

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Project description:Understanding arsenic reactions with ferric hydroxides is important in understanding arsenic transport in the environment and in designing systems for removing arsenic from potable water. Many experimental studies have shown that the kinetics of arsenic adsorption on ferric hydroxides is biphasic, where a fraction of the arsenic adsorption occurs on a time scale of seconds while full equilibrium may require weeks to attain. This research employed density functional theory modeling in order to understand the mechanisms contributing to biphasic arsenic adsorption kinetics. The reaction energies and activation barriers for three modes of arsenate adsorption to ferric hydroxides were calculated. Gibbs free energies of reaction depended on the net charge of the complexes, which is a function of the system pH value. Physical adsorption of arsenate to ferric hydroxide proceeded with no activation barrier, with Gibbs free energies of reaction ranging from -21 to -58 kJ/mol. The highest Gibbs free energies of reaction for physical adsorption resulted from negative charge assisted hydrogen bonding between H atoms on the ferric hydroxide and O atoms in arsenate. The conversion of physically adsorbed arsenate into monodentate surface complexes had Gibbs free energies of activation ranging from 62 to 73 kJ/mol, and Gibbs free energies of reaction ranging from -23 to -38 kJ/mol. The conversion of monodentate surface complexes to bidentate, binuclear complexes had Gibbs free energies of activation ranging from 79 to 112 kJ/mol and Gibbs free energies of reaction ranging from -11 to -55 kJ/mol. For release of arsenate from uncharged bidentate complexes, energies of activation as high as 167 kJ/mol were encountered. Increasingly negative charges on the complexes lowered the activation barriers for desorption of arsenate, and in complexes with -2 charges, the highest activation barrier was 65 kJ/mol. This study shows that the slow kinetics associated with arsenic adsorption and desorption can be attributed to the high Gibbs free energies of activation for forming and breaking bonds with the ferric hydroxide.

Dataset Information

Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction.

Publications

Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction.

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