Americans Robert Lefkowitz and Brian Kobilka won the 2012 Nobel Prize in chemistry Wednesday for studies of proteins that let body cells respond to signals from the outside.

The Royal Swedish Academy of Sciences said the two researchers had made groundbreaking discoveries on an important family of receptors, known as G-protein-coupled receptors.

About half of all medications act on these receptors, so learning about them will help scientists to come up with better drugs.

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

The Nobel Prizes were established in the will of 19th century Swedish industrialist Alfred Nobel, the inventor of dynamite. Each award is worth 8 million kronor, or about $1.2 million. The awards are always handed out on Dec. 10, the anniversary of Nobel's death in 1896.