Cofactors and Apo/Holoenzymes

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Cofactors and Apo/Holoenzymes

Post by mshah » Mon Oct 09, 2017 7:58 pm

Confused on the breakdown of cofactors..

Is it cofactors (metal, non-protein molecules that assist in catalytic activity) and then a subset of cofactors are coenzymes (organic carrier molecules), and then two types of coenzymes are cosubstrate (loosely bound to an enzyme) and prosthetic groups (tight bound to an enzyme)? Then holoenzyme is an enzyme with a cofactor and an apoenzyme is an enzyme without a cofactor? Thanks!
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Re: Cofactors and Apo/Holoenzymes

Post by NS_Tutor_Andrew » Mon Oct 09, 2017 10:42 pm

Hi mshah,

Thanks for the question! This terminology is definitely confusing. It sounds like you've basically got it, though. The only thing to note is that not all cofactors are metallic (because coenzymes are a subset of cofactors). Instead, "cofactor" is a blanket term that contains (1) inorganic (i.e., metallic) cofactors and (2) coenzymes, which are organic cofactors. Then, as you pointed out, coenzymes can be split into cosubstrates (loosely/transiently attached) and cofactors (tightly attached, often but not always through covalent bonds).

You've got the holoenzyme/apoenzyme distinction down. A useful mnemonic is that the holoenzyme is the whole thing.

Although not mentioned in your question, an additional point to note is that many vitamins are either coenzymes or the precursors to coenzymes. You should definitely know which vitamins are fat-soluble (A, D, E, K) and which are not (the others -- i.e., the B vitamins and vitamin C), and it's worth reviewing the basic functions of each vitamin. It's unlikely that the MCAT will test you on the details between, say, vitamin B6 and vitamin B9, but knowing the vitamins in general terms is definitely recommended.

Best of luck as you keep studying!!
Andrew D.
Content Manager, Next Step Test Prep.
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