Malate shuttle for gluconeogenesis

[ARSdogma]

It's from the EK 1001 Bio, lecture 1 #77.

Excerpt from passage 109:

"Oxaloacetate requires a malate shuttle to cross the inner mitochondrial membrane. Oxaloacetate is converted to malate, a TCA cycle intermediate, via reversal of the malate dehydrogenase reaction. A cytosolic isozyme of malate dehydrogenase is used to reform oxaloacetate in the cytoplasm"



"According to the information in the passage, which of the following statements concerning the malate shuttle is true?"

A) It is a peripheral mitochondrial membrane protein.
B) It requires hydrolysis of ATP.
C) It is an integral mitochondrial membrane protein.
D) Oxaloacetate binds to the receptor on the shuttle and gets transported across.

The correct answer is C.


I don't understand why. I took a guess because I saw the word shuttle and figured it's something the oxaloacetate binds to or I don't really know why. It was a complete guess.
The passage says Oxaloacetate requires a malate shuttle. WHAT IS a "malate shuttle"? What's a shuttle, i never heard that before.
And since the oxaloacetate is coming through to the cytosol from the mito, I am guessing it turns into malate or somehow has something to do with malate in order to get across the membrane, and then once in the cytosol the "cytosolic isozyme" turns the malate back into oxaloacetate??

How is that an integral protein it sounds like just conversion via enzymes?

Anyone can help me on this..

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[Danlee07]

Do you know your TCA cycle? If so, this question is simple. If not, inference will work. Let's take the inference approach.

From what you know about a cell and its intermembrane space, most of it is hydrophobic. The mitochondria is in this intermembrane space of the cell. From this knowledge alone, you should know it's either A or C. B is just a really dumb choice for pretty much any question. ATP hydrolysis is a totally different reaction, so it won't be a good choice unless it pertains to it... To eliminate it further, recall the difference between a protein/enzyme. Enzymes only catalyze reactions. They don't do anything to affect the concentrations, just the speed. Proteins are used for lots of things including shuttling through hydrophobic layers.

With the mitochondria being inside the intermembrane space and the the knowledge that something has to be shuttled inside, it can't really be on the outside of the mitochondria where tca takes place.

if you knew about the tca cycle, then you'd know L-malata to oxaloacetate is interchangeable...esp since it takes 2 cycles for it to get to the PDH complex. the reaction is just by malate dehydrogenase which contains proteins that are used for shuttling.