scholarly journals The mechanism of tyrosinase-catalysed oxidative decarboxylation of α-(3,4-dihydroxyphenyl)-lactic acid

1991 ◽  
Vol 277 (3) ◽  
pp. 849-853 ◽  
Author(s):  
M Sugumaran ◽  
H Dali ◽  
V Semensi
Keyword(s):  
Lactic Acid ◽  
Enzyme Reaction ◽  
Quinone Methide ◽  
Oxidative Decarboxylation ◽  
Mushroom Tyrosinase ◽  
Mechanistic Studies ◽  
Methyl Group ◽  
Enzymic Oxidation ◽  
Steady State Kinetic ◽  
State Kinetic

Mushroom tyrosinase, which is known to catalyse the conversion of o-diphenols into o-benzoquinones, has been shown to catalyse the oxidative decarboxylation of 3,4-dihydroxymandelic acid [Sugumaran (1986) Biochemistry 25, 4489-4492]. To account for this unusual reaction, a quinone methide intermediate has been proposed. Since all attempts to trap this intermediate ended in vain, mechanistic studies were designed to support the formation of this transient product. Replacement of the alpha-proton in 3,4-dihydroxymandelic acid with a methyl group generates alpha-(3,4-dihydroxyphenyl)-lactic acid, the enzymic oxidation of which should produce 3,4-dihydroxyacetophenone as the end product if the oxidative decarboxylation proceeds through the quinone methide intermediate. Accordingly, chemically synthesized alpha-(3,4-dihydroxyphenyl)-lactic acid on enzymic oxidation produced 3,4-dihydroxyacetophenone as the major isolatable product. Non-steady-state kinetic analysis of the enzyme reaction attested to the transient formation of the conventional quinone product. Thus the enzymic oxidation of alpha-(3,4-dihydroxyphenyl)-lactic acid seems to generate the conventional quinone, which, owing to its instability, is rapidly decarboxylated to yield the transient quinone methide. The coupled dieneonephenol re-arrangement and ketol-enol tautomerism transforms the quinone methide into 3,4-dihydroxyacetophenone.

scholarly journals Mechanistic studies on tyrosinase-catalysed oxidative decarboxylation of 3,4-dihydroxymandelic acid

1992 ◽  
Vol 281 (2) ◽  
pp. 353-357 ◽  
Author(s):  
M Sugumaran ◽  
H Dali ◽  
V Semensi
Keyword(s):  
Steady State ◽  
Visible Region ◽  
Quinone Methide ◽  
Spectral Studies ◽  
Oxidative Decarboxylation ◽  
Mechanistic Studies ◽  
Enzymic Oxidation ◽  
Steady State Kinetic ◽  
State Kinetic ◽  
Alpha Carbon

Mushroom tyrosinase, which is known to convert a variety of o-diphenols into o-benzoquinones, has been shown to catalyse an unusual oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde [Sugumaran (1986) Biochemistry 25, 4489-4492]. The mechanism of this reaction was re-investigated. Although visible-region spectral studies of the reaction mixture containing 3,4-dihydroxymandelic acid and tyrosinase failed to generate the spectrum of a quinone product during the steady state of the reaction, both trapping experiments and non-steady-state kinetic experiments provided evidence for the transient formation of unstable 3,4-mandeloquinone in the reaction mixture. The visible-region spectrum of mandeloquinone resembled related quinones and exhibited an absorbance maximum at 394 nm. Since attempts to trap the second intermediate, namely alpha,2-dihydroxy-p-quinone methide, were in vain, mechanistic studies were undertaken to provide evidence for its participation. The decarboxylative quinone methide formation from 3,4-mandeloquinone dictates the retention of a proton on the alpha-carbon atom. Hence, if we replace this proton with deuterium, the resultant 3,4-dihydroxybenzaldehyde should retain the deuterium present in the original substrate. To test this hypothesis, we chemoenzymically synthesized alpha-deuterated 3,4-dihydroxymandelic acid and examined its enzymic oxidation. Our studies reveal that the resultant 3,4-dihydroxybenzaldehyde retained nearly 90% of the deuterium, strongly indicating the transient formation of quinone methide. On the basis of these findings it is concluded that the enzymic oxidation of 3,4-dihydroxymandelic acid generates the conventional quinone product, which, owing to its unstability, is rapidly decarboxylated to generate transient alpha,2-dihydroxy-p-quinone methide. The coupled dienone-phenol re-arrangement and keto-enol tautomerism of this quinone methide produce the observed 3,4-dihydroxybenzaldehyde.

The mechanistic study of Leishmania major dihydro-orotate dehydrogenase based on steady- and pre-steady-state kinetic analysis

2016 ◽  
Vol 473 (5) ◽  
pp. 651-660 ◽  
Author(s):  
Renata A.G. Reis ◽  
Patricia Ferreira ◽  
Milagros Medina ◽  
M. Cristina Nonato
Keyword(s):  
Steady State ◽  
Leishmania Major ◽  
De Novo ◽  
Enzyme Reaction ◽  
Mechanistic Study ◽  
Rate Limiting Step ◽  
Steady State Kinetics ◽  
Steady State Kinetic ◽  
Rate Limiting ◽  
State Kinetic

Leishmania major dihydro-orotate dehydrogenase (DHODHLm) oxidizes dihydro-orotate to orotate (ORO) in the de novo pyrimidine biosynthetic pathway. The enzyme reaction mechanism was elucidated by steady- and pre-steady-state kinetics. ORO release was found to be the rate-limiting step in the overall catalysis.

scholarly journals Steady-state kinetic mechanism of bovine brain tubulin: tyrosine ligase

1992 ◽  
Vol 286 (1) ◽  
pp. 243-251 ◽  
Author(s):  
N L Deans ◽  
R D Allison ◽  
D L Purich
Keyword(s):  
Steady State ◽  
Thermal Inactivation ◽  
Enzyme Reaction ◽  
Kinetic Mechanism ◽  
Competitive Inhibitor ◽  
Product Inhibition ◽  
Maximal Velocity ◽  
Steady State Kinetic ◽  
Tubulin Tyrosine Ligase ◽  
State Kinetic

The ATP-dependent resynthesis of tubulin from tyrosine and untyrosinated tubulin was examined to establish the most probable steady-state kinetic mechanism of the tubulin: tyrosine ligase (ADP-forming). Three pair-wise sets of initial rate experiments, involving variation of two substrates pair-wise with the third substrate held at a high (but non-saturating) level, yielded convergent-line data, a behaviour that is diagnostic for sequential mechanisms. Michaelis constants were 14 microM, 1.9 microM and 17 microM for ATP, untyrosinated tubulin and L-tyrosine respectively, and the maximal velocity was 0.2 microM/min. AMP was a competitive inhibitor with respect to ATP, and a non-competitive inhibitor versus either tubulin or tyrosine. Likewise, L-dihydroxyphenylalanine acted competitively relative to tyrosine and non-competitively with respect to either ATP or tubulin. These findings directly support a random sequential mechanism. Product inhibition patterns with ADP were also consistent with this assignment; however, inhibition studies were not practical with either orthophosphate or tyrosinated tubulin because both were very weak inhibitors. Substrate protection of the enzyme against alkylation by N-ethylmaleimide and thermal inactivation, along with evidence of enzyme binding to ATP-Sepharose and tubulin-Sepharose, also supports the idea that this three-substrate enzyme reaction exhibits a random substrate addition pathway.

scholarly journals The EcoRI restriction endonuclease, covalently closed DNA and ethidium bromide

1981 ◽  
Vol 199 (3) ◽  
pp. 767-777 ◽  
Author(s):  
S E Halford ◽  
N P Johnson
Keyword(s):  
Steady State ◽  
Restriction Endonuclease ◽  
Ethidium Bromide ◽  
Enzyme Reaction ◽  
Transient Kinetics ◽  
Open Circle ◽  
Steady State Kinetic ◽  
Ecori Restriction ◽  
State Kinetic ◽  
Linear Product

The reactions of the EcoRI restriction endonuclease on the covalently closed DNA of plasmid pMB9 were studied in the presence of ethidium bromide. At the concentrations of ethidium bromide tested, which covered the range over which the DNA is changed from negatively to positively supercoiled, the dye caused no alteration to the rate at which this enzyme cleaved the covalently closed DNA to yield the open-circle form, but the rate at which these open circles were cleaved to the linear product could be inhibited. The fluorescence change, caused by ethidium bromide binding with different stoichiometries to covalently closed and open-circle DNA, provided a direct and sensitive signal for monitoring the cleavage of DNA by this enzyme. This method was used for a steady-state kinetic analysis of the reaction catalysed by the EcoRI restriction enzyme. Reaction mechanisms where a complex between DNA and Mg2+ is the substrate for this enzyme were eliminated, and instead DNA and Mg2+ must bind to the enzyme in separate stages. The requisite controls for this fluorimetric assay in both steady-state and transient kinetics studies, and its application to other enzymes that alter the structure of covalently closed DNA, are described.

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