US chemists Robert Lefkowitz and Brian Kobilka won the Nobel Prize on Wednesday for identifying a class of cell receptor, yielding vital insights into how the body works at the molecular level.

The award is for chemistry but the big beneficiary is medical research, the Nobel committee declared.

The pair were honoured for discovering a key component of cells called G-protein-coupled receptors and mapping how they work.

The receptors stud the surface of cells, sensitising them to light, flavour, smells and body chemicals such as adrenaline and enabling cells to communicate with each other.

About a thousand of these kinds of receptor are known to exist throughout the body. They are essential not just for physiological processes but also for response to drugs.

"About half of all medications achieve their effect through G-protein-coupled receptors," the Nobel jury said.

Understanding the receptors provides the tools for "better drugs with fewer side effects," Nobel committee member Sven Lidin said.

Lefkowitz, 69, is a professor of biomedicine and biochemistry at Duke University in North Carolina, while Kobilka, born in 1955, is a professor of molecular and cellular physiology at Stanford University School of Medicine in California.

In a teleconference with Swedish journalists, Lefkowitz admitted he had not heard the phone ring to get the famous piece of news.

"I was fast asleep and the phone rang. I did not hear it. I must share with you that I wear ear plugs to sleep, and so my wife gave me an elbow: 'phone for you.' And there it was. A total shock and surprise," he said.

Lefkowitz admitted that his day had been thrown out of whack.

"As yet we've told nobody," he said. "I plan to go to the office. I was going to get a haircut, which if you could see me is quite a necessity, but I'm afraid (that) will probably have to be postponed.

"I think it'll be a crazy day at the office."

Kobilka meanwhile told Swedish news agency TT he was also awakened in the middle of the night at his home in California.

Asked if he would be able to fall back to sleep, he replied: "I don't think so."

"I'm still very surprised, they called me just an half hour ago, but now it is starting to slowly sink in," he said.

Kobilka said he had not yet decided what he would do with his half of the eight million Swedish kronor ($1.2 million, 930,000 euros) prize sum.

"I don't know. I have two children and hopefully they will inherit some of it. I'm not really used to my work being recognised like this," he added.

Mark Sansom, a professor of molecular biophysics at the University of Oxford, said the receptors "have for a long time been the holy grail of membrane protein research."

"They are fundamental to regulation of many physiological processes, from the nervous system to taste and smell," he told the Science Media Centre in London.

"They are also a major class of drug target and are incredibly important to the pharmaceutical industry."

On Monday, Shinya Yamanaka of Japan and John Gurdon of Britain won the Nobel Medicine Prize for work in cell programming, a frontier that has raised dreams of replacement tissue for people crippled by disease.

On Tuesday, the physics prize went to France's Serge Haroche and David Wineland for research in quantum physics that could one day open the way to supercomputers.

The literature prize will be announced on Thursday, followed Friday by perhaps the most-watched award, for peace. The economics prize wraps up the Nobel season on Monday.

The laureates will receive their prizes at formal ceremonies in Stockholm and Oslo on December 10, the anniversary of prize founder Alfred Nobel's death in 1896.

[Illustration of G protein-coupled receptor via]