6. The affinity or dissociation constant (Kd) between a protein and a small mole

Question

6. The affinity or dissociation constant (Kd) between a protein and a small molecule was measured using equilibrium dialysis at 25, 30, and 37°C. The Kd values were 9.9x10,1.6 x 10*, and 3.12 x 105 M respectively. (a) How is Kd related to Kcq? (b) Is the binding between the protein and the small molecule better or worse with increasing temperature? Explain your answer. (c) Using excel or a similar program, make a van't Hoff plot. Include a table showing any results from calculations, a trendline, and an equation of best fit. (d) What are the corresponding , and AS°, for formation of the protein-small molecule complex? (e) At 37°C degrees, is complex formation favorable? If so, is it driven by entropy, enthalpy or both? Assume that , and So, are constant with respect to temperature.

Explanation / Answer

Ans 1.kd is the inverse of keq.

For eg. P +L=PL

Keq=[PL] /[P] [L]

Kd=[P] [L] /[PL]

So keq at 25'C will be 1/9.9X10^-8=1.01X10^7

Similarly keq at 30°C =6.25X10^5

Keq at 37°C=3.2X10^4

2 . The binding between protein and small molecules will be worse with increasing the temperature because heat increases the kinetic energy which causes the molecules to vibrate rapidly and fastly that disrupts the bonds like hydrogen bonds and non-polar hydrophobic interactions.

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