Americans Robert Lefkowitz and Brian Kobilka have been awarded the 2012 Nobel Prize in Chemistry.

The Royal Swedish Academy of Sciences in Stockholm announced the winners of the prestigious prize on Wednesday.

Lefkowitz and Kobilka won for their research on G-protein coupled receptors, a part of the cell which affects how pharmaceutical drugs are absorbed by the body.

Lefkowitz, a professor at Duke University in North Carolina, said he was “shocked and surprised” upon hearing the news, adding that he was asleep when the call came in.

Kobilka is a professor at Stanford University in California.

Nobel prize announcements began Monday with the medicine prize going to stem cell researchers John Gurdon of Britain and Japan's Shinya Yamanaka. Frenchman Serge Haroche and American David Wineland won the physics prize on Tuesday for work on quantum particles.

The Nobel prize for literature will be announced on Thursday, and the prestigious Peace Prize on Friday.

The Nobel Memorial Prize in economic sciences will be announced on October 15.