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Swastika posted an Question
December 24, 2021 • 12:43 pm 30 points
  • IIT JAM
  • Biotechnology (BT)

An enzyme requires both aspartate (pka of side chain = 4.5) and histidine (pka of side chain = 6.5) residues in the catalytic site to be protonated for activity

An enzyme requires both aspartate (pKa of side chain = 4.5) and histidine (pKa of side chain = 6.5) residues in the catalytic site to be protonated for activity. The expected enzyme activity (in %) at a pH of 5.5 would be closest to A 90 B. 78 C. 50 D. 10 all'c

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  • Swastika

    in the answer key its showing that option D is correct

  • Priya sarda best-answer

    90% PKa=4.5 and PKb=6.5 Therefore, PI = 4.5+6.5÷2=5.5 The optimum pH depends on the amino acid components of active site. The Isoelectric point depends of the full enzyme primary and tertiary structure. Particularly I've never seen a relationship between these two properties. The pI and pH optimum are not related. To put it simply, at the pH optimum, the pH is such that, some amino acids groups are positively charged, and some are negatively charged, so that  the substrate can bind to the active site through electrostatic interactions. More importantly, at the pH optimum, the charges (positive or negative) on certain amino acids (catalytic residues) are critical, for the actual enzymatic reaction or catalysis to take place. so when pI = pH the activity of enzyme will be max. Note: No enzyme works at rate of 100%.

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