Saint John's vs national capital

Saint John's and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
Saint John's	Yes	No	No	No
national capital	Yes	No	No	No

Saint John's (noun)	national capital (noun)
a port and provincial capital of Newfoundland	the capital city of a nation
the capital and largest city of Antigua and Barbuda; located on the island of Antigua