By Joe Brock

JOHANNESBURG (Reuters) - South African Paralympic gold medalist Oscar Pistorius will be sentenced this week for the 2013 murder of his girlfriend Reeva Steenkamp, ending years of legal twists and turns.

Pistorius, 29, known as "Blade Runner" for the carbon-fiber prosthetic blades he used to race, faces a minimum 15-year jail sentence and cannot appeal after the country's top court ruled in March that he had exhausted all his legal options.

The track star, whose lower legs were amputated when he was a baby, initially received a five-year sentence for culpable homicide, South Africa's equivalent of manslaughter. The conviction was later upgraded to murder after an appeal heard by the Supreme Court.

Original trial judge Thokozile Masipa will begin hearing pre-sentencing arguments at Pretoria High Court on Monday, with Pistorius expected to discover his fate by the end of the week.

The athlete's legal team is expected to call witnesses who will argue that Masipa should be lenient because of the athlete's mental fragility, physical disability and good behavior during almost a year behind bars for the original manslaughter conviction.

State prosecutor Gerrie Nel will cross-examine and call his own witnesses as he seeks to convince Masipa that Pistorius is not remorseful -- a key consideration in sentencing -- and that it is in the interest of South African justice that the athlete receives a lengthy jail term.

Pistorius could take the stand himself, as could Reeva's father Barry Steenkamp, who has given a number of emotional interviews calling for justice that he says would not be served if Pistorius is treated leniently.

The athlete's legal team did not respond to phone calls for comment. State prosecutors said they would not disclose who their witnesses would be.

Pistorius's final days in court will mark the end of a dramatic fall from grace for an athlete who was once considered a heroic example of triumph over adversity.

The sprinter became the first double amputee to compete against able-bodied athletes when he raced at the London 2012 Olympics after a five-year fight for the right to run on equal terms.

He was released from prison last October after almost a year behind bars and allowed to serve out his term under house arrest on his uncle's property in a suburb of Pretoria.

But in December, the Supreme Court upgraded the conviction on appeal and Pistorius was allowed to stay at his uncle's mansion pending the final decisions on appeals and sentencing.

Pistorius denies deliberately killing model and law graduate Steenkamp, saying he mistook her for an intruder when he fired four shots through a locked toilet door in his Pretoria home.

(Editing by David Goodman)