the tris runs between pH 7.2 and 9, if you have to test pH outside this range, must use other buffers

  eg. citrate-phosphate-borate (Teorrel and Stenhagen) whose pH range is from 2 to 12.

good luck

Thank u @ Claudia Sartori.What if the enzyme should be assayed in particular buffer. Say for an example collagen suspended in 50 mM TES buffer. Do you suggest me to use the citrate-phosphate-borate buffer system?

You are completely right, that changing the buffer might severely change the activity of an enzyme. As Claudia already mentioned, Tris buffers optimally between 7.2–9.0. Therefore, I would suggest that you limit your study to a pH range of about 7.0-9.2. Most likely, you'll find a maximum in this range. Why should you examine the areas of poor activity in more detail? In most cases, this is of no further interest.

The important thing to remember is that buffers are only effective at pH values within 1 unit of the pKa value. For Tris, pKa 8.1 the useful working range is 7.1-9.1, but the buffering power is very much less at the extremes. In your studies you have two key parameters, pH and buffer type. You also introduce a third variable if you try other buffers (ionic strength).

Unless you know that the pH optimum is near pH 8.6 you may need to look at a wider range of pH values (and thus a wider range of buffers).  Unless you know that Tris is the best buffer you could try other buffers at pH 8.6 (e.g. EPPS). For lower pH values acetate (pH 5) Mes (pH 6), MOPS pH 7.2, Hepes, pH 7.5 are good choices. You can also look at the effect of changing buffer at a fixed pH. e.g. both MES and MOPS each could reasonably be used at pH 6.5. Mes and acetate could each be used at pH 5.5. 

The triple buffer systems suggested above are easier to use in some ways, but you introduce three chemicals (thus potentially three inhibitors) into your assay. It is far more common to use single buffer systems; the pKa of the chosen buffer is normally within 0.5 units of the assay pH. Your current assay set up is within these guidelines. 

Every enzyme is different and I have seen ten fold differences in activity simply by changing the type of buffer at constant pH. There is no reason why Tris should be the best buffer option, it just tends to be readily available in most labs. Finally the pH optimum is likely to be near the pH at which the enzyme normally works, though even this is not guaranteed!

Thank you @ Michael G. Weller. I prefer the pH in the physiological range..I just gone curious to go beyond this pH range and to find out whether my enzyme with stands that kinda pH

Thank you @Nick Gee..

I agree with Claudia Sartori, but be care with the temperature, especially in the range where the buffer pH is being controlled by the Tris  Tris buffers can easily shift by 0.5 of a pH unit in response to temperature between 15-40C. Therefore adjust/set  the buffer pH at the experimental temperature.

In order to determine the optimum pH correctly "zero buffer extrapolation method" should be used thus the effect of buffers on the enzyme is eliminated. Any buffer selected between pKa 4-10 should be used between the 1 pH unit above and below of its pKa value. First, a graph of different concentrations of buffer at certain pH is prepared and that pH and buffer concentration activity is measured. The activity versus buffer concentrations graph is plotted and the activity at "zero buffer concentration" was found by extrapolation. Second graph is plotted using the activities at zero buffer concentrations versus pHs. Tangents to lower and upper pH parts of the activity versus pH curve are extended and from the intersection point of the two lines a vertical line drawn to the pH coordinate will give correct pH optimum.

The buffer you use to fix pH is not indifferent. That is two different buffers at the same nominal pH might give you very different enzyme activities.

I suggest you read this:

http://www.sciencedirect.com/science/article/pii/S1359029416300486

If you can't access I can send you the file.

Dear Yogi Affable,

You have not stated what pH dependence you want to study. It could be

-       the temperature dependence of he enzyme activity at a fixed pH

-       the pH dependence of the enzyme activity (from which you can also determine the pH dependence of kcat and Km) in the pH range where the enzyme is stable

-       the pH dependence of the enzyme stability by determining the rate of enzyme denaturation as a function of pH

the buffers you use must be used 1 pH unit above and below their pK-values as indicated by Nick Gee and Ozer Nazmi above. A detailed list of suitable biological buffers in the pH range is given in

https://www.applichem.com/fileadmin/Broschueren/BioBuffer.pdf

where you also will find extensive information on the buffers but also that the enzyme activity also depends on the ionic strength I, and reference to important primary papers. It is however not explicitly stated that TRIS is not a suitable buffer for  enzymes that form covalent acyl-enzyme intermediates (hydrolases, transferases). The acylated enzyme can be deacylated by the uncharged TRIS amino group. 

Thus for simple Michaelis Menten kinetics the measured reaction rate v is

V(pH, T, I) = [EH] kcat(pH, T, I) [S]/(Km(pH, T, I) + [S])

where [EH] and [S] are enzyme and substrate concentration.

It makes no sense, and would be very time consuming,  to determine enzyme activities at zero buffer content by extrapolations as Ozer Nazmi suggests.

To study the pH dependence of the enzyme the temperature and the ionic strength must be kept constant in the studied pH range. In the internet you find a free suitable tool to prepare buffers of different pHs with constant ionic strengths.

Google for

Buffer Calculator – BioMol.Net.

A recent open access source on recommendations for  reporting enzyme data in the scientific literature, given in several excellent papers, you will find by googling for

Reporting Enzymology Data – STRENDA Recommendations and Beyond.

Very beautiful question. Go through the almanac that you have or get one from a nearby store. Look out for the stars, the moon and the position of sun with respect to all the planets in our and other solar systems and find out the most auspicious moment. At that moment prepare any buffer you like and it should work for you pH studies. Do not tell this secret to anybody. Keep it only between you and me. 

During assays you want to keep as many conditions constant as possible, and change - ideally - only one. Thus, you would try to keep the buffer ions constant and change only the pH. However, buffers work only near the pKa of the buffering ion, in the case of Tris that is 8.07 at 25 °C with a temperature coefficient of -0.03 units per °C. In other words, you can use Tris at most in the pH-range 7-9. Some multivalent buffers like citrate have a broad buffering range because the pKa-values are 3.13, 4.76 and 6.39. Thus, citrate can be used between pH 2.1 and 7.4 (look up Gomori-buffer).