Is Constitution a proper noun

No, the noun 'constitution' is a common noun, a general Word for a system of laws and principles that prescribes the functions and limits of a government or another institution; a general Word for the document that describes this system; a general Word for the form or structure of something; a general Word for the physical health and condition of a person or animal.

A common noun is capitalized only when it's the first Word in a sentence.

A proper noun is the name or title of a specific person, place, or thing; for example, The Constitution of the United States of America or Constitution Avenue in Washington DC.