Johannesburg vs urban center

Johannesburg and urban center both are nouns.

Word	Noun	Adjective	Verb	Adverb
Johannesburg	Yes	No	No	No
urban center	Yes	No	No	No

Johannesburg (noun)	urban center (noun)
city in the northeastern part of South Africa near Pretoria; commercial center for diamond and gold industries	a large and densely populated urban area; may include several independent administrative districts