Hi!
I have attached my answers under your text (which I have quoted in italics)
In the AAMC prep, passage 7 question 53 asks essentially what would happen if a lactose molecule were to be hydrolyzed based on passage information. The passage shows us an image of a compound turning into a beta D-galactose molecule.
My question is: in a hydrolysis of a lactose molecule, how can I determine if the Oxygen will be added onto my galactose or my glucose molecule. Galactose and glucose are very structurally similar molecules other than the change of a hydroxyl group to an opposite side.
How can we deduce that a deprotonated water molecule will attack galactose and not glucose?
Often, the chemistry passages might tell you in english exactly what is happening for a reaction, and you have to extract that information when reading. This sounds deceptively simple but can be quite tricky to do, especially at a 2nd year ochem level.
For this passage, the first paragraph is describing the mechanism of lactose hydrolysis. Note the last sentence: Glu-1 deprotonates water, and the resulting hydroxide ion acts as a nucleophile to liberate galactose and regenerate the enzyme.
Here, because it states that water attacks galactose, if we had 18 O labelled water we would expect galactose to be labelled, eliminating A and B.
Furthermore, we know earlier that Glu-2 is the nucleophile that liberates D-glucose, not water. Therefore glucose would not be labelled in this way, and we can eliminate D, leaving us with option C. Please let me know if this is unclear and I can draw the mechanism the first paragraph is describing.
Essentially though, if the passage refers to a mechanism you do not know, it is likely described in the passage. Good passage notes and mapping make this type of question easier (and practice helps).
In addition, question 54 asks "For what mechanistic reason does G1 of lactase first act as a Brønsted acid during catalysis". Is G1 notation of sugars? Nowhere in this passage is this notation mentioned. Additionally, in relation to my above question, this question requires us to know that in the lactose molecule, glucose is acting as a leaving group. How would we be able to tell that glucose acts as the leaving group? Does glucose always act as a leaving group in a hydrolysis of a Lactose reaction?
Recall that lactase is the enzyme that hydrolyzes lactose ( -ase is a suffix indicative of hydrolase enzymes in general). I don't love this question because they are using G to refer to the glutamates (even though the one letter code of glutamate is E), so it is understandably confusing on your end as this question should probably be revised with Glu or E.
Anyways, back to the question - how you know glucose is acting as a leaving group - based on the statement that Glu-2 acts as a nucleophile to liberate D-glucose and generate an a-D-galactopyranosyl modified enzyme intermediate mentioned in paragraph one, we can deduce that this is an SN2 like reaction (you have a nucleophile attacking and something leaving to form a new compound with the nucleophile attached).
As each Sn2 reaction needs a leaving group, we know that glucose leaves, which eliminates D as an answer choice. From there, we realise Glu-1 acts as an acid, not a nucleophile, eliminating A, and Glu-2's protonation state doesn't change in the Glu-1 protonation, eliminating B, and leaving us with C as the most likely answer.
Again, this shows one of the bigger challenges of the chem section - reading dense reaction mechanism descriptions and translating them to answer questions.
Please let me know if this is unclear and I can elaborate.