MCAT Biology Question -- Amino Acids | Next Step Test Prep MCAT Biology Question -- Amino Acids | Next Step Test Prep

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The amino acid histidine has a nitrogen-containing side chain. The pKa for the carboxylic acid group, imidazole side chain, and the primary amine has a value of 1.8, 6.0, and 9.0, respectively.

 

histidine

 

Which of the following structures accurately reflects the predominant form of histidine at a pH of 4.0?

corrected_blog_answer_histidine_9_10_2014

 

 

Explanation

To answer this question, we need to find out whether each of the three acidic or basic groups on histidine will be protonated or deprotonated. These groups are the carboxylic acid terminal, the amino terminal, and the imidazole side chain.

To determine the charge state of each group, compare its pKa to the surrounding pH (4.0). If the pH is greater (more basic) than the pKa, the group will be deprotonated, since groups tend to lose protons under basic conditions. If the pH is less (more acidic) than the pKa, the group will be protonated.

Let’s begin with the carboxylic acid group. The pH (4.0) is greater than the pKa (1.8), so the group will be deprotonated, making it negative (COO). Next, both the imidazole side chain and the amino terminal have pKas that are greater than the pH, so these groups will be protonated. Only option C shows a deprotonated carboxylic acid terminal and protonated side chain and amino terminal.

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