As everyone knows:
Vmax = Kcat x [E]t
with [E]t is total enzyme CONCENTRATION
Vmax unit eg umol.min-1 (Im sorry, I mistyped this before)
Kcat unit should be min-1 or s-1
So my question is, if the [E]t is really a concentration, we can never get the unit for Kcat as min-1 or s-1 but there always the volume unit appears along with the time unit, such as ml.ml-1.
However, if you use the unit mol for [E], you will get the unit for Kcat as s-1 or min-1.
This raises to me a question that do people really mean [E] as the concentration with the unit e.g. mol.l-1 or do they mean [E] as the AMOUNT of enzyme with the unit of mole only?
Thank you.
Much of the misunderstanding about units employed for describing enzyme activity derives from the practice (which has a long history) of expressing activity in terms of AMOUNT of product formed per unit time (e.g. micromole/min) instead of using the RATE of product formation (e.g. micromole/ml per min). In chemical reaction kinetics, reaction rates are always considered. Why enzymology still uses amount/min is difficult to explain,.but it is probably too late to change things now. As long as authors are aware of the paradox there is no cause to worry. Some confusion can arise however when authors use 'mol' as an abbreviation when they probably mean mol/litre
Peter
Your unit for Vmax is wrong, Vmax is the maximal change in concentration per time unit: Vmax is a speed, e.g. umol*ml-1*min-1, [E] a concentration, e.g. umol*ml-1
Vmax is a rate, and has units of inverse time built into it; its units are molarity/time, e.g. mol/L-s or mol/L-min. [E] is a concentration, i.e. units of mol/L.
Oh, sorry, I mistyped it! It's umol.min-1
When you use this unit, and [E] as concentration, umol.ml-1 for example, you cannot get rid of the volume unit in the final unit of Kcat.
Here is what I found: http://chemwiki.ucdavis.edu/Biological_Chemistry/Catalysts/Enzymatic_Kinetics/Turnover_Number
They use the amount of enzyme instead of concentration.
I guess, we use the amount of enzyme or the concentration depends on the unit of Vmax we use.
The units at that link you provided are misleading, in a way. Units of volume were omitted from both Vmax and [E], simply because they cancel out. Kcat is always expressed in units of inverse time, (i.e. molecules per unit time), and is volume independent.
Yes, that is what I was a bit confused since the symbol [E] means concentration of E. So when people use Vmax in the unit of eg mol.min-1 then we automatically assume E as the amount of enzyme instead of the concentration. But in some textbook, they still put [E] even when the unit for Vmax they are using is like above which makes it confusing.
Vmax can also be expressed as eg mol.min-1.ml-1 but in some exercises, they only say mol.min-1 which also confused me at first.
I got it now! Thank you very much for your help.
The units at the link are correct. The units of kcat are moles of product/sec divided by moles of enzyme. In consequence, kcat resulted in 1/time units. Enzyme concentration can be M, mM, uM, etc. The result of kcat*[E] is rate units: mM/min, uM/s, etc.
Much of the misunderstanding about units employed for describing enzyme activity derives from the practice (which has a long history) of expressing activity in terms of AMOUNT of product formed per unit time (e.g. micromole/min) instead of using the RATE of product formation (e.g. micromole/ml per min). In chemical reaction kinetics, reaction rates are always considered. Why enzymology still uses amount/min is difficult to explain,.but it is probably too late to change things now. As long as authors are aware of the paradox there is no cause to worry. Some confusion can arise however when authors use 'mol' as an abbreviation when they probably mean mol/litre
Peter
Totally agreed with Ms.@Annemarie Honegger .
and also, Vmax is based on the substrate catalysed or product formed.Its precise to mention in these terms than the E concentration.
correct me if I'm wrong.
[E]: I agree with D. Cohen however we need to be more precisely: it is the concentration of ACTIVE enzyme that should correctly be used here. Frequently authors speak of total protein in the reaction, this is not the active protein which can slightly less to several orders of magnitide less (depending on the enzyme purity and the ratio of active to inactive protein). If you are using total protein, then you need to indicate this in your tables/graphics, thus for example your will not be measuring a specific activity, but a pseudo-specific activity which can differ with each and every purification, whereas for a given assay and given conditions the specific activity is a constant
The correct is that [E] is a concentration. This is out of any discussion. The important question is: What are the units for [E]? We should use the molar concentration units. However, it is not an easy task to determine it. That's why the Internation Units of enzyme activity are used: to avoid the determination of molar concentrations of enzyme. Another option is the use of mass/volume units (g/ml; g/l; g/kg) for practical consideration, for example, in industrial applications. In this case, the units of kcat must be congruent with the units of [E]. I recommend to take a look to the book Enzyme Biocatalysis: Principles and Applications, by Andres Illanes.
Can i plot then the Michaelis Menten graph using concentration units expressed in mass/volume?
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