Next Step Full Length 10, Bio/Biochem Passage 9

Session 130

We’re rounding out bio/biochem for Blueprint MCAT (formerly Next Step Test Prep) full-length 10 and we’re in for a great passage to help us with our skills.

[01:54] Passage 9

Ketone bodies are molecules that are produced by the liver during times of low glucose levels. [Figure 1 is talking about the structures of the three naturally occurring ketone bodies in humans – acetone, acetoacetic acid, and beta-hydroxybutyric acid] These ketone bodies are synthesized from acetyl-CoA.

Normally in the liver, acetyl-CoA enters the Krebs cycle. However, if there is a lack of oxaloacetate or an overabundance of acetyl-CoA released from beta-oxidation of fatty acids, then the acetyl-CoA will be built in the ketone bodies via ketogenesis.

If acetoacetate is not used within about five hours of its creation, it would be decarboxylated into a metabolically inert ketone. Acetyl acetate and beta-hydroxy butyryl dehydrogenase are picked up from the liver and transported to other parts of the body where they can be converted back into acetyl-CoA and enter the Krebs cycle.

The brain and heart are the biggest consumers of ketone bodies as the source of energy. The state of higher than normal ketone body levels is known as ketosis. Significantly high levels of ketone bodies will lower a blood pH, a condition known as ketoacidosis.

[03:35] Question 48

Which of the following best explains why high blood levels of ketone bodies lower blood pH?

(A) Because acetoacetic acid and beta-hydroxybutyric acid have PKA values above physiological pH.
(B) Because production of ketone bodies produce CO2 which reacts with water to form carbonic acid.
(C) Because production of ketone bodies releases Coenzyme A which is highly acidic.
(D) Because acetoacetic acid and beta-hydroxybutyric acid have PKA values below physiological pH.

Clara’s insights:

Lower PKA corresponds to something more acidic just like a lower pH is more acidic. Answer choice A would have been backward as PKA above would be less acidic. Then B and C may look tempting as they involved acid in some way but we don’t need to know that kind of detail about ketone body production. So the answer here is D.

[07:52] Question 49

Would a person with diabetes be at a greater risk of developing ketoacidosis?

(A) Yes, because lack of insulin means a lack of glucose in liver cells halting the Krebs cycle and causing an overabundance of acetyl-CoA resulting in ketogenesis and ketoacidosis.
(B) No, because lack of insulin means an abundance of glucose in blood, meaning there’s plenty of glucose available and at the end of the Krebs cycle is pyruvate.
(C) No. Lack of insulin means lack of glucose in liver cells. However, the Krebs cycle will not be interrupted even without pyruvate.
(D) Yes, because lack of insulin means an abundance of glucose in blood resulting in higher acetyl-CoA levels which then undergo ketogenesis resulting in ketoacidosis.

Clara’s insights:

You may not know what abundance of glucose in the blood would do but you can kind of guess what it will do in cells. Glucose in the blood doesn’t really do anything. And we picture all these processes just happening but they actually have to happen in cells. So the abundance of glucose in the blood is in no way will directly result in higher acetyl-CoA levels because glucose would need to be in the cells for it to go through glycolysis and make pyruvate to progress into the Krebs cycle. So A is the correct answer.

[12:52] Question 50

Based on the passage, what is the most likely path for removal of acetone produced in Figure 1.

Excretion in the urine
Conversion back to acetyl acetate

III. Involved in anabolism of amino acids

(A) I only.
(B) I and II only.
(C) II and III only.
(D) I, II, and III

Clara’s insights:

When you say III, it means it’s used as a building block for amino acids so that’s not right. Then the conversion back to acetyl acetate, it doesn’t necessarily say that acetone comes from acetyl acetate. So the correct answer here is I only.

The passage doesn’t directly don’t talk about acetone but it alludes to it in a way we can figure out what it’s talking about. So if you actually look back at paragraph 3, it says “If acetoacetate is not used within about five hours of its creation, it would be decarboxylated into a metabolically inert ketone.” Acetone is a ketone. Acetoacetate is a pretty small molecule where essentially the only ketone you get from it is acetone. So when they say metabolically inert ketone, the obvious identity for that is acetone. And if it’s metabolically inert, it doesn’t really do anything in metabolism and we just excrete it out.

And amino acids are small, like individual monomeric building blocks. We wouldn’t imagine like a ketone body would be used to build those tiny little amino acids anyway.

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