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Ornithine transcarbamylase catalyzes the reaction shown.

$$\text{ornithine + carbamyl phosphate} \ \xrightarrow [ ]{ } \ \text{citrulline}$$

In order to assay this enzyme, the enzyme is mixed with 0.1 M Tris, pH 8.5, 0.1 M ornithine pH 8.5 and 0.2 M carbamyl phosphate for 20 minutes. At that time, 10% perchloric acid is added.

In a second step, a portion of this is removed and a mixture is added, boiled, and a purple color is formed that can be detected based on its absorbance at 540 nm.

In an enzyme assay, the activity is measured in units and the value of 1 unit is
Select Option 1 mole citrulline/minute1M citrulline/min1 micromole citrulline/min1 $\mu M$ citrulline/min
. In the enzyme assay for ornithine transcarbamylase, the role of the 0.1 M Tris pH 8.5 solution is
Select Option as a substrate for enzymeto provide a constant pH for the enzymeto stop the reaction by denaturing the enzyme
, and the 0.1 M ornithine pH 8.5 is added to
Select Option provide a constant pH for the enzymestop the reaction by denaturing the enzymeact as a substrate for the enzyme
. Since this assay required a second step to produce the purple color, this enzyme assay is an example of
Select Option a direct enzyme assayan indirect enzyme assayneither a direct nor an indirect enzyme assay
.
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