TBR Questions

[Saigon]

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Hi, since TBR is my primary content review I will just make a thread and keep posting whenever I have a new question. I have the newest edition of the books.

Right now I have 2 questions from the TBR Bio 7 (metabolic components).

freestanding #13: How many different isozymes can be formed, if the tetrameric enzyme lactate dehydrogenase has two subunits, A and B?
A. 2
B. 3
C. 4
D. 5

Answer is 5, but I have know idea how to get to this answer. I tried drawing out different combinations of the dimers but I was confused as to which ones were/weren't legit or the same.


My second question: One TBR question asked which of the following metabolic processes did not produce metabolic water: protein formation from amino acids, triacylglycerol breakdown, last step of the ETC, and glycogen synthesis. I got the answer right (fat breakdown) but I am trying to pinpoint where water is produced during glycogenesis.

 

[Hospitalized]

Hi, since TBR is my primary content review I will just make a thread and keep posting whenever I have a new question. I have the newest edition of the books.

Right now I have 2 questions from the TBR Bio 7 (metabolic components).

freestanding #13: How many different isozymes can be formed, if the tetrameric enzyme lactate dehydrogenase has two subunits, A and B?
A. 2
B. 3
C. 4
D. 5

Answer is 5, but I have know idea how to get to this answer. I tried drawing out different combinations of the dimers but I was confused as to which ones were/weren't legit or the same.

Interesting. I just looked it up, wikipedia has the exact enzyme and it shows the dimers!

https://en.wikipedia.org/wiki/Isozyme

 

[rhinzaz]

Hey Saigon,

Great questions;

As for your first Q, the answer lies in understanding that lactate dehydrogenase is a tetrameric enzymes with subunits A and B. Therefore; you can write out the different combinations of A/B that constitute up four subunits, or one tetrameric enzyme; you should come up with something like;
AAAA
ABAA
ABBA
ABBB
BBBB

The above would be 5 different ways to form a tetrameric enzyme using 2 subunits.

Your reasoning regarding trying to draw 'different combinations of the dimers' leads me to believe you're trying to relate it to hemoglobin structure; where it's a 'dimer of a dimer (ABAB)', which is a special type of isozyme - not all isozymes are dimer dimers.

Your second question requires a bit of organic chemistry thinking (some arrowpushing may help). When glucose-glucose bonds are formed during glycogenesis via 1-4-alpha-glycosidic linkages, think about what happens to the OH groups from the 1' and 4' carbon of the 2 original glucose and what the final bond looks like. This should remind you of a simple dehydration synthesis (where water is a byproduct).

 

[Saigon]

Thank you very much for taking the time to respond. Your answer to the first part makes sense. I definitely understand in a normal glycosidic bond the sugars combine and release water. I actually just took a year of biochem and maybe I am overthinking it/they wanted the most wrong answer? If my memory serves me correct the steps for glycogenesis are the following steps, none of which lead to water liberation.

Glucose + ATP -> Glucose 6-phosphate + ADP
Glucose 6-phosphate -> Glucose 1-phosphate
Glucose 1-phosphate + UTP -> Glucose-UDP + PPi (Ppi + H2O -> 2Pi)
Glucose-UDP + Glycogen/Glycogenin -> Glycogen + UDP

 

[wizzed101]

I thought all TBR questions have explanations?

Anyway, I think you missed the point of the 2nd question. The question was something like this:

Which of the following processes does not produce water?

A. An anabolic process
B. A catabolic process
C. Last reaction of the electron transfer chain (O2 + e+ H+ -> H20). This you should know.
D. An anabolic process

Catabolic rxn generally requires hydrolysis and anabolic reactions are most of the time condensation rxns which produce water. You have 1 minute to answer the question and the question will require around 1 minute to figure out. If you are doing too much work, you are doing it wrong.

And the 4th rxn on your list will liberate water.

 

[Saigon]

Not all of the TBR questions have in-depth explanations for why each answer was rejected. How does the 4th reaction liberate water? The Glycogen hydroxyl will attack the glucose-udp and the hydrogen on that hydroxyl will be added to the udp alkoxide.

 

[wizzed101]

I referred to the branching reaction.

If you don't have branches, you don't have glycogen

 

[Saigon]

TYTYTY! I completely forgot about the branching enzyme 🙂 Your the best. I owe you one!

 

[physiciantobe]

Hey Saigon,

Great questions;

As for your first Q, the answer lies in understanding that lactate dehydrogenase is a tetrameric enzymes with subunits A and B. Therefore; you can write out the different combinations of A/B that constitute up four subunits, or one tetrameric enzyme; you should come up with something like;
AAAA
ABAA
ABBA
ABBB
BBBB

The above would be 5 different ways to form a tetrameric enzyme using 2 subunits.

Your reasoning regarding trying to draw 'different combinations of the dimers' leads me to believe you're trying to relate it to hemoglobin structure; where it's a 'dimer of a dimer (ABAB)', which is a special type of isozyme - not all isozymes are dimer dimers.

Your second question requires a bit of organic chemistry thinking (some arrowpushing may help). When glucose-glucose bonds are formed during glycogenesis via 1-4-alpha-glycosidic linkages, think about what happens to the OH groups from the 1' and 4' carbon of the 2 original glucose and what the final bond looks like. This should remind you of a simple dehydration synthesis (where water is a byproduct).

What about combinations like BAAB or BAAA? Why is it only those specific combinations? Was this a question where you just had to know that LDH exists in five forms?

 

[Oorham]

H

Hey Saigon,

Great questions;

As for your first Q, the answer lies in understanding that lactate dehydrogenase is a tetrameric enzymes with subunits A and B. Therefore; you can write out the different combinations of A/B that constitute up four subunits, or one tetrameric enzyme; you should come up with something like;
AAAA
ABAA
ABBA
ABBB
BBBB

The above would be 5 different ways to form a tetrameric enzyme using 2 subunits.

Your reasoning regarding trying to draw 'different combinations of the dimers' leads me to believe you're trying to relate it to hemoglobin structure; where it's a 'dimer of a dimer (ABAB)', which is a special type of isozyme - not all isozymes are dimer dimers.

Your second question requires a bit of organic chemistry thinking (some arrowpushing may help). When glucose-glucose bonds are formed during glycogenesis via 1-4-alpha-glycosidic linkages, think about what happens to the OH groups from the 1' and 4' carbon of the 2 original glucose and what the final bond looks like. This should remind you of a simple dehydration synthesis (where water is a byproduct).

Doesn't it have to have both subunits A and B in its tetrameric structure? Also, as CallMeDoc96 mentioned what about the other structures like BABB, BAAB, BAAA?