Question 23

[M.C.]

Hi,
Couldn't C also be an answer to this question due to the presence of pepsin in the stomach?
Thank you

[matthew]

In this problem, basic knowledge of properties of the alpha amino acids would help. The passage mentions that phenylalanie(with a CH3 group attached to phenyl) hydroxylase is required for metabolism of amino acid W to amino acid Y (tyrosine), so this enzyme (-ase) is making the -CH3 group into an -OH group. All the other answers don't say this.

[admin]

Good points Matthew though you do not need to memorize amino acids for the MCAT.

Yes, pepsin is in the stomach: it is a protease, meaning an enzyme which cleaves proteins into smaller subunits. It has optimal activity at low pH like that of the stomach. I think M.C. you meant to ask that because the stomach is so acidic, wouldn't the excess protons protonate any -OH group?

Yes, but that is not related to the passage. The passage discusses a liver enzyme which means it will be secreted into the bile duct (exocrine), merge with the pancreatic juices down the common bile duct into the duodenum with lots of bicarbonate to neutralize any acid coming from the stomach. Thus the duodenum (first stage of the small intestine) is not acidic unless you are looking for an ulcer! You did not need to have known the configuration of these amino acids but you are responsible for knowing the basic physiology of digestion (BIO 9.4).

[admin]

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