Marathon Petroleum Corp said it raised the cash portion of its offer to buy MarkWest Energy Partners LP, the second-largest U.S. natural gas processor, by $400 million.

The company, which is buying MarkWest through its pipeline unit, MPLX LP, said it will now offer $5.21 per unit in cash, up from the $3.37 it offered in July.

MarkWest unitholders will continue to get 1.09 MPLX units for each share held, the company said on Nov. 10.