"It was not a big bang, it was a thud but the impact was massive. The attacker was apparently walking behind us," he said.
Sherpao's son also suffered minor injuries in the attack.
One of Sherpao's aides and several of his security detail were killed, and police said they had found the head and torso of the bomber.
Asif Iqbal Daudzai, information minister for the provincial government, said: "We have got the severed head of the bomber, and it is identifiable. He appears to be between 30-35 years old,"
Kamal Hyder, Al Jazeera's Pakistan correspondent, said Sherpao had faced heavy criticism for his handling of the security situation in country.
He had also made enemies among the Afghan population for forcing many of them out of the country, Hyder said.
The border province has a large community of Afghan refugees, most living in camps.