Commissioned in 2286, the USS Enterprise-A was a refit of the Constitution Class Starship, becoming an Enterprise Class Starship1

The Enterprise-A was commanded by Captain Kirk, with first officer Commander Spock until decommissioned in 2293.

The two most notable missions undertaken by the Enterprise-A are the journey to the center of the galaxy after being captured by Sybok (Laurence Luckinbill) at Nimbus III in 2331 (Stardate 8454.1) (Star Trek V: The Final Frontier) and failing to escort Chancellor Gorkon (David Warner) to the Khitomer Peace Conference in 2293, though the crew did manage to stop the assassination of the President of the United Federation of Planets. (Star Trek VI: The Undiscovered Country)

The USS Enterprise-A was an MK IX Heavy Cruiser with a crew of approximately 430 and had a max spead of Warp 9.5. (old warp scale)

1 An interesting note: The USS Enterprise (NCC-1701) was a Constitution Class Starship thus making Enterprise Class Starships perhaps the only class named after a ship of a different class. This class is also called the Constitution II Class.

Additionally, in Star Trek VI: The Undiscovered Country Scotty views a blueprint of the "Enterprise" and it's labeled "Constitution Class."