Consider the molecule below. If pK_1 = 2.4 and pK_2 = 11.3, what is its net char

ID: 96362 • Letter: C

Question

Consider the molecule below. If pK_1 = 2.4 and pK_2 = 11.3, what is its net charge at pH 12? The figure depicts the primary structure of an enzyme. The amino acids that make up the active site are marked for the wild type enzyme (top amino acids) and a mutant enzyme (bottom amino acids). Based on this, what is your BEST prediction about the mutant enzyme? T F ? Mutant enzyme will have lower activity: Ile will not bind substrate as well as Leu, and will not stabilize the transition state. T F ? Mutant enzyme will have lower activity: mutation will cause protein to unfold. T F ? Mutant enzyme will have the same activity as wild type: Leu and Ile have similar R-groups with similar weak bonds.

Explanation / Answer

Answer: 08. The net charge determination for an amino acid is depends on the pH values. For that reason we need to look into the ionizable groups and their values for the given amino acid. Above the structure is given for valine.

Ionizable groups

pKa

Caboxy-terminal group (COOH)

Approx 2.0

Amino Terminus (NH3+)

Approx 11.3

amino group (NH3+)

Approx 12

With Valine at 2.4 pKa the net charge is +1. The N-terminus, for pKa 11.3 is -1, at 12 = -1, so the pH 12 will have net value of = -1.

09. Leucine and isoleucine both the amino acids are in the same group, as they are ‘non-polar, aliphatic’. They also have the same side chains or R groups, only difference is the position for their side groups is different. Similarly we need to remember here is that their side chains are non reactive or non-polar their binding activity does not changes. They are also not involved in covalent chemistry.

In chemical change in proteins often in biological system we could see that isoleucine, an essential amino acid having hydrocarbon side chain is serving the interchangeable position with leucine. However the beta- carbon of isoleucine is optically active, it is very important for ligand binding to proteins and to make the protein chain stable also.

From the option above we have these options as false, as the change of leucine to isoleucine does not weaken the bond. i) Mutant enzyme will have lower activity, ii) Mutant enzyme will have lower activity; mutation will cause protein to unfold.

We have to choose the right answer as iii) Mutant enzyme will have the same activity as wild type; leucine and isoleucine have similar weak bonds, as they are non-polar.