MCAT Chemical and Physical Foundations of Biological Systems 5 — Flashcards

Questions are based on the following passage:

Ribonuclease L (RNase L) is a key antiviral endoribonuclease that is induced and activated by virus infection, interferons, or double-stranded RNA (dsRNA). A cofactor called 2-5A binds to monomeric RNase L to convert it to an active, dimeric enzyme. When active, dimeric RNase L degrades single-stranded RNA to eliminate virus and virus-infected cells. Curcumin, an orange-yellow pigment from the herb a.longa, has been found to have numerous therapeutic effects. The chemical structure is shown in Figure 1. Its effect on RNase L activity was studied. The interaction was determined to follow Michaelis-Menten kinetics.


Reaction mixtures were prepared that contained purified protein, 2-5A, and 0, 5, or 10 µM curcumin. RNA degradation was quantified by measuring the intensity of the residual intact bands of 28S and 18S rRNAs in each reaction and subtracting it from that of a background control reaction containing no 2-5A cofactor.

Using this method, Michaelis-Menten curves were created to test the effect of curcumin on RNase L activity (Figure 2). With curcumin present, the Km did not change, but Vmax decreased. Finally, the change in intrinsic fluorescence intensity [(F0-F)/F0] from a 280-nm excitation wavelength at the 340-nm emission wavelength was measured with increasing curcumin concentrations in order to test the ability of the molecule to bind RNase L .

The Kd value for binding was determined by fitting the curve using the Hill equation.


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