AKIPRESS.COM - Mustafa Koc, the 55-year-old chairman of Turkish conglomerate Koc Holding and one of the country's most prominent businessmen, died on Thursday after a heart attack.

Head of the pre-eminent dynasty of Turkey's secular business elite, Koc has had, at times, an uneasy relationship with the Islamist-rooted AK Party founded by President Tayyip Erdogan, reports Reuters.

Recently there had been signs of reconciliation, however, and sources from the presidential palace said the two had met on Wednesday evening, hours before Koc died.

Koc's businesses, which range from energy and automobiles to banking, account for close to 10 percent of Turkey's national output and include five of its 10 largest companies.

Erdogan called Koc's father, Rahmi Koc, and his brother, Ali Koc, to express his condolences, saying the magnate's death had "saddened him deeply", presidential palace sources said.

Koc collapsed while exercising and received emergency treatment from a trainer and a guard, the Beykoz state hospital, where he was first taken, said.

It said his heart was not beating when he arrived and efforts to resuscitate him began before he was transferred by helicopter to the American Hospital, a facility run by the Koc family foundation, in central Istanbul.

"Despite all the efforts of the doctors, we lost Mustafa Koc as a result of a heart attack which he suffered at home," an official told reporters outside the American Hospital.

Shares in Koc fell as much as 5 percent after the news of his death, and were down 2.8 percent by later afternoon, while the main Istanbul share index was 1.5 percent weaker.

U.S. Ambassador John Bass, writing on his Twitter account, described Koc's death as "a great loss for Turkey and all his friends around the world".