Biochemistry question Bank Query

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aru
Posts: 5
Joined: Thu Jul 05, 2018 12:41 pm

Biochemistry question Bank Query

Post by aru » Thu Dec 06, 2018 7:59 am

The enzyme adenylate kinase catalyzes the following reaction: (question 40 of biochemistry question bank metabolism)
ADP + ADP ↔ ATP + AMP
The reverse process predominates when the cell is using ATP at a low rate. Which of these species likely downregulate(s) the activity of one or more glycolytic enzymes?
I. AMP
II. ATP
III. ADP
The answer is both ATP and ADP. Iunderstand that ATP is inhibitor of hexokinase, PKF and pyruvate kinase in addition to other inhibitors like citarate, glucose-6-phosphate and acetyl CoA also inhibits the glycolytic pathway as well. How is ADP an inhibitor/allosteric down regulator of glycolysis. If high ADP is present doesn't that mean cell is in demand of energy? I don't understand the explanation provided. Can you please help me with this.
Thank you.
NS_Tutor_Will
Posts: 121
Joined: Fri May 25, 2018 9:15 am

Re: Biochemistry question Bank Query

Post by NS_Tutor_Will » Thu Dec 06, 2018 10:57 am

Hi Aru,

It sounds like you have a solid understanding of ATP's role here, so I'll focus on ADP.

We need to draw on the information presented in the question stem to answer this question. First, note that under low ATP-consuming conditions, the reverse of the reaction show will predominated. So, we will have a higher concentration of ADP when energy consumption is low. (And vice versa -- when we are using lots of energy, the reaction will go the other way, so we'd expect ATP and AMP to be favored.)

So: When ADP concentrations are high, we are consuming less energy. Thus, it's reasonable to conclude that ADP regulates an enzyme involved in metabolic energy generation (glycolytic, in this case).

Note that you really do need to use the information presented in the Q stem. to get to this answer.

I hope this helps and good luck!
aru
Posts: 5
Joined: Thu Jul 05, 2018 12:41 pm

Re: Biochemistry question Bank Query

Post by aru » Fri Dec 07, 2018 4:42 pm

Hi,
Thank you for the explanation. Can you explain me the difference between phosphofructokinase-1 and phosphofructokinase-2. Are they both isoform of the same enzyme? Additionally can you explain me more about the bifunctional activity of PKF2 and how it regulates the glycolytic and gluconeogenetic pathways.
Thanks.
NS_Tutor_Will
Posts: 121
Joined: Fri May 25, 2018 9:15 am

Re: Biochemistry question Bank Query

Post by NS_Tutor_Will » Sun Dec 09, 2018 9:01 am

PFK-1 and PFK-2 are indeed isoenzymes. PFK-2 basically helps to activate PFK-1 when conditions are such that glycolysis is proceeding rapidly, or there is inhibition of PFK-1 by ATP, citrate, or some other substrate of metabolism.

The reason PFK-2 is referred to as "bifunctional" is because it has two active domains and can thus regulate multiple pathways. One is a kinase, while the other is a phosphatase. In glycolysis, PFK-2 helps activate PFK-1 (thus promoting the cycle). In gluconeogenesis, PFK-2 inhibits fructose-1-6-biphosphatase 1.

Thanks for your question. I wouldn't get too worried about this high-level metabolism stuff because it's not very likely that you'll see it on your actual MCAT. I usually advise students to focus on understanding metabolism in a conceptual way because that is usually the type of understanding that can apply across multiple questions, passages, and sections.
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