Let's deal with names first. Names denote individuals; nouns - common mouns as opposed to proper nouns, i.e. names, - denote classes/categories.
I am assume that in the question
"Who are you, the biochemist?"
you are using 'biochemist' as a category label and not as the name of an individual, as in
a) 'Who are you, Mary?'
- speaking to someone called Mary
b) 'Who are you, Frenchman?'
- speaking to a Frenchman, whose name you do not know, or
c) 'Who are you, Biochemist?'
-speaking to a biochemist whose name you do not know.
If you were using 'biochemist' as a name in a direct question, this form of address would be rude, perhaps even aggressive. If you wished to use 'biochemist as a name or title, you would also use a capital 'B without an article.