scholarly journals Influence of the position of the double bond in steroid substrates on the efficiency of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo steroid Δ4–Δ5-isomerase

1980 ◽  
Vol 185 (3) ◽  
pp. 723-732 ◽  
Author(s):  
Hadassa Weintraub ◽  
Etienne-Emile Baulieu ◽  
Annette Alfsen
Keyword(s):  
Double Bond ◽  
Proton Transfer ◽  
Transfer Reaction ◽  
Proton Transfer Reaction ◽  
Process Data ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Isomerization Process ◽  
Rate Limiting ◽  
Pseudomonas Testosteroni

Studies of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo steroid Δ4–Δ5-isomerase with Δ5(6)- and Δ5(10)-steroid substrates demonstrate the importance of the position of the double bond for the efficiency of the isomerization process. Thus 3-oxo-Δ5(6)-substrates have markedly high kcat. values, whereas those of 3-oxo-Δ5(10)-substrates are very low and their apparent Km values approach equilibrium dissociation constants. The first step in the isomerization process is: [Formula: see text] which is governed by the k−1/k+1 ratio and is shown to be very similar for the two classes of substrates (3-oxo-Δ5(6)- and -Δ5(10)-steroids). They therefore differ in the steps distal to the initial formation of the Michaelis–Menten complex. The use of the deuterated androst-5(6)-ene-3,17-dione substrate enabled us to calculate individual rate constants k+1 and k−1 as well as to determine the apparent rate-limiting step in the isomerization process. With the deuterated oestr-5(10)-ene-3,17-dione substrate, no significant isotope effect was observed suggesting that a different rate-limiting step may be operative in this isomerization process. Data are presented that indicate that under optimal concentrations of the efficient androst-5(6)-ene-3,17-dione substrate, the forward reaction for ES complex formation (as defined by k+1) is limited only by diffusion and the apparent Km does not approach the equilibrium constant, suggesting that the evolution of this enzyme has proceeded close to ‘catalytic perfection’.

scholarly journals Atomically manipulated proton transfer energizes water oxidation on silicon carbide photoanodes

2018 ◽  
Vol 6 (47) ◽  
pp. 24358-24366 ◽  
Author(s):  
Hao Li ◽  
Huan Shang ◽  
Yuchen Shi ◽  
Rositsa Yakimova ◽  
Mikael Syväjärvi ◽  
...  
Keyword(s):  
Silicon Carbide ◽  
Electron Transfer ◽  
Proton Transfer ◽  
Water Oxidation ◽  
Proton Coupled Electron Transfer ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

Preferential exposure of Si-face of SiC will mechanistically shift the rate limiting step of water oxidation from sluggish proton-coupled electron transfer on C-face to a more energy-favorable electron transfer.

Base-catalyzed formation of spiro adduct from N-methyl-N-(2,4,6-trinitrophenyl)glycinamide, the smiles rearrangement of the amide in methanol

1988 ◽  
Vol 53 (3) ◽  
pp. 601-618 ◽  
Author(s):  
Jaromír Kaválek ◽  
Vladimír Macháček ◽  
Makky M. M. Hassanien ◽  
Vojeslav Štěrba
Keyword(s):  
Proton Transfer ◽  
Equilibrium Constant ◽  
Ammonium Salt ◽  
Aliphatic Amine ◽  
Reaction Pathway ◽  
Smiles Rearrangement ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting ◽  
Kinetics Of

The reaction of N-methyl-N-(2,4,6-trinitrophenyl)glycinamide (Ic with methoxide in methanol produces the spiro adduct IIc(A). In methanolic acetate buffers, the equilibrium is rapidly established between the spiro adduct IIc(A) and the dipolar ion of 2-methylamino-N-(2,4,6-trinitrophenyl)acetamide (IIIc(Z)). The equilibrium constant of the reaction IIIc(Z) ⇆ IIc(A) + H+ is by eight orders of magnitude greater than that of the analogous cyclization of 2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)acetamide to the spiro adduct. In chloracetate buffers, the dipolar ion is protonated to give 2-methylammonium-N-(2,4,6-trinitrohenyl)acetamide IIIc(K). The kinetics of the reversible reaction IIIc(Z) ⇆ IIc(A) + H+ has been studied in acetate buffers, aliphatic amine – ammonium salt buffers, and methoxide solutions. In all cases, the rate-limiting step was the proton transfer with half-lives in milliseconds. In more basic methanolic buffers (pH > 10) the rate-limiting step consists in the formation of spiro adduct from the zwiterion IIIc(Z) resulting from the protonation of the anion IIIc(A). n acetate buffers, the second reaction pathway via the cation IIIc(K) is predominant.

scholarly journals Substrate-Dependent Sensitivity of SIRT1 to Nicotinamide Inhibition

Biomolecules ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 312
Author(s):  
Stacia Rymarchyk ◽  
Wenjia Kang ◽  
Yana Cen
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Negative Regulator ◽  
Base Exchange ◽  
Glucose And Lipid Metabolism ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Catalyzed Reactions ◽  
Solvent Isotope ◽  
Rate Limiting

SIRT1 is the most extensively studied human sirtuin with a broad spectrum of endogenous targets. It has been implicated in the regulation of a myriad of cellular events, such as gene transcription, mitochondria biogenesis, insulin secretion as well as glucose and lipid metabolism. From a mechanistic perspective, nicotinamide (NAM), a byproduct of a sirtuin-catalyzed reaction, reverses a reaction intermediate to regenerate NAD+ through “base exchange”, leading to the inhibition of the forward deacetylation. NAM has been suggested as a universal sirtuin negative regulator. Sirtuins have evolved different strategies in response to NAM regulation. Here, we report the detailed kinetic analysis of SIRT1-catalyzed reactions using endogenous substrate-based synthetic peptides. A novel substrate-dependent sensitivity of SIRT1 to NAM inhibition was observed. Additionally, SIRT1 demonstrated pH-dependent deacetylation with normal solvent isotope effects (SIEs), consistent with proton transfer in the rate-limiting step. Base exchange, in contrast, was insensitive to pH changes with no apparent SIEs, indicative of lack of proton transfer in the rate-limiting step. Consequently, NAM inhibition was attenuated at a high pH in proteated buffers. Our study provides new evidence for “activation by de-repression” as an effective sirtuin activation strategy.

scholarly journals DFT study on the mechanism of simultaneous trifluoromethylation and oximation of aryl-substituted ethylene.

2021 ◽  
Author(s):  
Sen Wang ◽  
Ao He ◽  
Xuan Meng ◽  
Xiao wei Lan ◽  
Xianfu Wei ◽  
...  
Keyword(s):  
Carbon Atom ◽  
Proton Transfer ◽  
Organic Solvent ◽  
Solvent Effect ◽  
Aromatic Ring ◽  
Organic Solvents ◽  
Energy Barrier ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting

The effects of different substituents located at the para position of the aromatic ring and β carbon atom of the styrene on the reaction were investigated. The results showed that the reaction steps with higher energy barriers changed a little with the substituents of the reactants, which indicates that the reaction has a good adaptability to reactants containing different substituents. It was found the proton transfer in the final tautomerism step of nitroso intermediate to oxime is the rate limiting step under anhydrous conditions. Although the solvent effect did not influence the the rate limiting step significantly, the water mediated proton transfer significantly decreased the energy barrier of final tautomerism step. Compared with the direct proton transfer in vacuum, the energy barrier of the final tautomerism step decreased from 57.80kcal/mol in vacuum to 12.98kcal/mol with the water mediated proton transfer in water, which declined by 77.5%. When water participates in rate-limiting steps in organic solvents, the energy barrier also decreases significantly, which indicates that a small amount of water in the organic solvent is conducive to the reaction. This study is of great significance for the application of bifunctionalized reaction in the synthesis of organic fluoride compounds with different substituents.

Kinetic Isotope Effects on the Rate-Limiting Step of Heme Oxygenase Catalysis Indicate Concerted Proton Transfer/Heme Hydroxylation

2003 ◽  
Vol 125 (52) ◽  
pp. 16208-16209 ◽  
Author(s):  
Roman Davydov ◽  
Toshitaka Matsui ◽  
Hiroshi Fujii ◽  
Masao Ikeda-Saito ◽  
Brian M. Hoffman
Keyword(s):  
Proton Transfer ◽  
Heme Oxygenase ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting
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