A method for producing disulfide bond containing proteins in prokaryotic cells, which comprises the steps of expressing in the cytoplasm of a prokaryotic cell (i) protein(s) of interest that naturally contain one or more disulfide bonds, and iia) naturally occurring transmembrane enzyme vitamin-K oxidoreductase (naturally occurring inverted VKOR), wherein the cysteines of the active site(s) are naturally located towards the prokaryotic cytoplasm and said enzyme is capable of catalysing the process of native disulfide bond formation, or iib) inverted transmembrane enzyme, wherein the cysteines of the active site(s) are after genetic engineering located towards the prokaryotic cytoplasm and said enzyme is capable of catalysing the process of native disulfide bond formation, said enzyme being selected from the group of a. inverted vitamin-K oxidoreductase VKOR (iVKOR) and b. inverted disulphide bond forming enzyme B (iDsbB), wherein genetic engineering of said DsbB or VKOR comprises the following steps of - removing one or more of the cytoplasmic lysine and/or arginine residues of DsbB or VKOR, and/or - fusing DsbB or VKOR to a transmembrane helix or transmembrane helices of any transmembrane protein, and (iii) cytoplasmic disulphide bond forming enzyme A (DsbA) being capable of providing electrons to the active site(s) of naturally occurring inverted VKOR, iVKOR or iDsbB whereby natively folded disulfide bond containing protein(s) of interest are formed in the prokaryotic cytoplasm; and whereby the charge distribution of the said enzyme in the membrane is changed and the said active site(s) of said DsbB or VKOR protein is shifted from the periplasmic space towards the prokaryotic cytoplasm. (i) protein(s) of interest that naturally contain one or more disulfide bonds, and iia) naturally occurring transmembrane enzyme vitamin-K oxidoreductase (naturally occurring inverted VKOR), wherein the cysteines of the active site(s) are naturally located towards the prokaryotic cytoplasm and said enzyme is capable of catalysing the process of native disulfide bond formation, or iib) inverted transmembrane enzyme, wherein the cysteines of the active site(s) are after genetic engineering located towards the prokaryotic cytoplasm and said enzyme is capable of catalysing the process of native disulfide bond formation, said enzyme being selected from the group of a. inverted vitamin-K oxidoreductase VKOR (iVKOR) and b. inverted disulphide bond forming enzyme B (iDsbB), wherein genetic engineering of said DsbB or VKOR comprises the following steps of - removing one or more of the cytoplasmic lysine and/or arginine residues of DsbB or VKOR, and/or - fusing DsbB or VKOR to a transmembrane helix or transmembrane helices of any transmembrane protein, and a. inverted vitamin-K oxidoreductase VKOR (iVKOR) and b. inverted disulphide bond forming enzyme B (iDsbB), wherein genetic engineering of said DsbB or VKOR comprises the following steps of - removing one or more of the cytoplasmic lysine and/or arginine residues of DsbB or VKOR, and/or - fusing DsbB or VKOR to a transmembrane helix or transmembrane helices of any transmembrane protein, and - removing one or more of the cytoplasmic lysine and/or arginine residues of DsbB or VKOR, and/or - fusing DsbB or VKOR to a transmembrane helix or transmembrane helices of any transmembrane protein, and (iii) cytoplasmic disulphide bond forming enzyme A (DsbA) being capable of providing electrons to the active site(s) of naturally occurring inverted VKOR, iVKOR or iDsbB