I am trying to find data regarding kinetic parameters of enzymes involved in a biochemical pathway. Km data is easily available, but I couldnt find Vmax data anywhere. (even in databases like Brenda).
Dear
Look the link. may be useful.
http://www.ucl.ac.uk/~ucbcdab/enzass/substrate.htm
http://www1.lsbu.ac.uk/water/enztech/determination.html
http://www.indiana.edu/~l113/independent_projects/addlprocedures/AP-VEnzymes.pdf
Regards, Shafagat
Hi there,
Vmax is not a constant as it directly dépends on the concentration of active enzyme in the assay you perform (Vmax=kcat*E0 for instance for michaelian enzyme transforming one substrate). You have to prefer kcat instead of Vmax as it is a real constant.
Vmax (just as kcat) is very variable and depends on the enzyme preparation and the mode of testing. Different degrees of purity or the presence of various amounts of inactivated enzyme affect the values to a degree that makes it very hard to compare two values taken in different labs (or even the same lab at different times or with different enzyme batches). Otherwise, Vmax (unit: micromol/(min mg protein) is basically the same as kcat (unit 1/sec). You obtain kcat by multiplying Vmax by the molecular mass (in g/mmol or kDa) of your protein (or the part of the protein representing a single enzymatically active unit for multi-subunit complexes) and converting min into sec. kcat then gives you the turnover-number of substrates per second, either by the entire enzyme or per active subunit.
Vmax is not available for many enzymes because its determination is too hard. To determine Vmax value, you have to run many reactions to measure the initial velocity at different concentrations of the substrate. Also, you can't calculate Kcat without Vmax value since Kcat = Vmax/enzyme concentration.
So, Kcat is an absolute constant and Vmax is not absolute since it is dependent upon the enzyme concentration. You can look for any review about the enzyme kinetics.
@Johann Heider:
You wrote "Vmax (just as kcat) is very variable". This is wrong: kcat is a constant which is defined for each enzyme/substrate couple.
You also wrote "Vmax (unit: micromol/(min mg protein) is basically the same as kcat (unit 1/sec)". This is also wrong for 2 reasons : unit for Vmax is a quantity of product formed (mol or mol/L) per time unit so there is no notion of mg of protein in it (which is found in the expression of specific activity). kcat and Vmax have different units so they definitely can't be the same.
You finally wrote "You obtain kcat by multiplying Vmax by the molecular mass (in g/mmol or kDa) of your protein ". This is also wrong as Vmax=kcat*[E]0, kcat is the ratio between observed Vmax expressed in mol/L/min and the concentration of enzyme in mol/L in the assay (if Vmax is in mol/min then enzyme has to be expressed in number of moles present in the assay)
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