[writer]

I want to put Coenzyme Q10 into one of the regular nutrient categories.

PubChem has it as a vitamin
https://pubchem.ncbi.nlm.nih.gov/compound/5281915#section=Other-Identifiers

It obviously is not a vitamin, because no CooQ10 deficiency is known.

Other sources have it as a provitamin, which sounds legit. To which vitamin CoQ10 can be converted?

[Babcock_Hall]

"Coenzyme Q10 (CoQ10) is an antioxidant that is made in the human body. CoQ10 is needed for basic cell function. CoQ10 levels decrease with age and may be low in people with cancer, certain genetic disorders, diabetes, heart conditions, HIV/AIDS, muscular dystrophies, and Parkinson's disease. Some prescription drugs may also lower CoQ10 levels."  http://www.mayoclinic.org/drugs-supplements/coenzyme-q10/background/hrb-20059019

"Primary dietary sources of CoQ10 include oily fish (such as salmon and tuna), organ meats (such as liver), and whole grains. Most people get enough CoQ10 through a balanced diet, but supplements may help people with particular health conditions (see Uses section), or those taking certain medications (see Interactions section)."  https://umm.edu/health/medical/altmed/supplement/coenzyme-q10

This compound is a member of the electron transport chain.

[Yggdrasil]

I don't think it would be classified as a provitamin.  CoQ10 is fully synthesized in the body, so it would not be able to help synthesize vitamins that the body cannot produce on its own. 

[writer]

CoQ10 is a quinone similar to vitamin K and can maybe be converted to it, but yes, this may be not enough to classify it as a provitamin. I'll probably just put it under "other nutrients," since quinones or antioxidants are really not nutrient categories.

[Yggdrasil]

Just because their structures are similar does not mean that the human body contains the enzymes required to convert CoQ10 to vitamin K.  For example, in the organisms that synthesize vitamin K, CoQ10 is present, but it is not used as a precursor to vitamin K.

[writer]

Yeah, I do not want to push. Another aspect is that CoQ10 is not essential nutrient anyway, so I do not want relate it with vitamins.

http://www.nutrientsreview.com/other/coenzyme-q10.html