21-hydroxylation of steroids

Abstract

Generally, the present invention relates to the field of steroid hydroxylation. More specifically, the present invention relates to a method for the 21-hydroxylation of steroids in cells. It also relates to cells expressing a steroid 21-hydroxylating enzyme or steroid 21-hydroxylase, expression vectors comprising a nucleic acid encoding for a steroid 21-hydroxylase and a kit for carrying out the method for the 21-hydroxylation of steroids in cells.

Claims

1. A process for the hydroxylation of the carbon atom 21 of a steroid, comprising the steps of: (a) providing a cell expressing: (i) a heterologous CYP21A2 protein or a functional variant thereof; (ii) at least one heterologous electron transfer system capable of transferring electrons to CYP21A2; and (iii) one or more chaperones facilitating folding of CYP21A2; and (b) adding the steroid to the cell, wherein the steroid is medrane or deltamedrane.

2. The process of claim 1, further comprising a step (c) of extracting the 21-hydroxylated steroid from the supernatant of the cell.

3. The process of claim 1, further comprising adding one or more cell permeabilizing agents to the cell after step (b).

4. The process of claim 1, wherein the cell is a resting cell.

5. The process of claim 1, wherein the cell is a prokaryotic cell or a eukaryotic cell.

6. The process of claim 1, wherein the at least one heterologous electron transfer system comprises: (a) a CYP21A2 reductase, and/or (b) a ferredoxin reductase.

7. The process of claim 1, wherein the one or more chaperones are recombinantly expressed chaperones.

8. The process of claim 1, wherein the expression of at least one tryptophanase gene is reduced or abolished in the cell.

9. The process of claim 1, wherein the cell further expresses a heterologous gene encoding for an enzyme catalyzing a step in the heme biosynthesis pathway.

10. The process of claim 1, wherein the genes encoding for (i), (ii), and optionally (iii) are comprised in one or more expression cassettes which are integrated into the cell genome.

11. The process of claim 5, wherein the cell is an E. coli cell.

12. The process of claim 5, wherein the cell is a yeast cell.

13. The process of claim 9, wherein the heterologous gene encoding for an enzyme catalyzing a step in the heme biosynthesis pathway is a hemA gene.

14. The process of claim 6, wherein the at least one heterologous electron transfer system comprises an NADPH-dependent ferredoxin reductase and a ferredoxin.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1: Hydroxylation of a steroid (here progesterone) at carbon atom 21.

(2) FIG. 2: Scheme of a whole cell biotransformation of a steroid by CYP21A2 and the needed electron transfer proteins in E. coli.

(3) FIG. 3: left: CO difference spectrum of purified bovine CYP21; right: SDS-PAGE of bCYP21 samples taken after indicated purification steps (IMAC/DEAE/SP).

(4) FIG. 4: HPLC chromatogram of the in vitro 21-hydroxylation of medrane to premedrol by human CYP21 and described electron transfer proteins, here AdR and Adx (system 2).

(5) FIGS. 5A-5B: Constructed vectors for whole cell biotransformation using human or bovine CYP21 with different electron transfer proteins.

(6) FIGS. 6A-D: HPLC chromatogram of the whole cell 21-hydroxylation of medrane to premedrol (A), delta-medrane to medrol (B), medroxyprogesterone to 21OH-medroxyprogesterone (C), and 17OH-progesterone to 11-deoxycortisol (D) by bovine CYP21 and described electron transfer proteins, here Fpr and Adx.

(7) FIG. 7: Time-dependent whole cell conversion of medrane to premedrol by bovine CYP21 and described electron transfer proteins, here Fpr and Adx.

(8) FIGS. 8A-8B: Amino acid sequences of wildtype and modified human (A) and bovine (B) CYP21A2.

DESCRIPTION OF THE EXAMPLES

Example 1: In Vitro Hydroxylation

(9) 1.1 Expression/Purification of hCYP21/bCYP21

(10) To show that both human and bovine CYP21 are able to hydroxylate steroids at position 21, in vitro studies with both enzymes were performed. As an exemplary 21-hydroxylation process, medrane was converted to premedrol:

(11) ##STR00001##

(12) Premedrol (methylhydrocortisone) is a precursor of a highly effective pharmaceutical steroid medrol (methylprednisolone). Medrol is an important drug in therapy of autoimmune diseases, multiple sclerosis and in general for local and systematic treatment of inflammations.

(13) Both enzymes were expressed in the E. coli strain C43(DE3) by coexpression of the E. coli chaperones GroEL/GroES encoded in the vector pGro12. These chaperones ensure a correct protein folding which is important for an incorporation of the heme prosthetic group. In FIG. 3 an SDS-PAGE and a CO difference spectrum of purified bovine CYP21 are shown. As the CO difference spectrum shows, the enzyme was purified in an active form. To determine the binding of medrane to both isozymes the binding constants (K.sub.D-values) were determined.

(14) 1.2 Expression of Electron Delivering Redox Partners

(15) For an efficient substrate conversion, both isoforms require an electron transfer system which consists of two parts, the cytochrome P450 enzyme itself and one or two electron transfer proteins which are essential for a hydroxylation reaction. Without these transfer proteins, no reaction will take place. Electrons can be transferred to CYP21 for example by the six electron transfer systems listed in Table 2:

(16) TABLE-US-00002 TABLE 2 Electron delivering proteins applied in CYP21-dependent substrate conversions and corresponding expression plasmids for whole-cell systems. hCYP21 or bCYP21 were combined in reconstituted systems or whole-cell systems with the indicated redox partners bCPR (bovine NADPH-dependent cytochrome P450 reductase), bAdR (bovine adrenodoxin reductase), bAdx.sub.4-108 (bovine adrenodoxin), Fpr (E. coli flavodoxin reductase), Arh1 (S. pombe adrenodoxin reductase homolog), etp1.sup.fd (S. pombe electron transfer protein, ferredoxin domain). Protein combinations in reconstituted in vitro systems Corresponding plasmids Reductase Ferredoxin in whole-cell systems 1 CPR p21h_bRED/p21b_bRED 2 AdR Adx.sub.4-108 p21h_AdAx/p21b_AdAx 3 Fpr Adx.sub.4-108 p21h_FrAx/p21b_FrAx 4 Arh Adx.sub.4-108 p21h_ArAx/p21b_ArAx 5 Arh etp1.sup.fd p21h_ArET/p21b_ArET 6 Fpr etp1.sup.fd p21h_FrET/p21b_FrET

(17) For in vitro studies and a verification of a substrate conversion, all redox partners were purified.

(18) 1.3 Reconstitution of Cytochrome P450 Systems In Vitro

(19) In vitro substrate conversions with purified enzymes in a defined buffer and with an NADPH regeneration system revealed that both isoforms together with the here listed electron transfer proteins are able to convert medrane to premedrol very efficiently. FIG. 4 shows the in vitro conversion of medrane by human CYP21 together with electron transfer system 2. This result indicates that steroids as exemplified by premedrol can be produced enzymatically by CYP21 together with a suitable redox system, e.g. as shown in Table 2.

Example 2: Whole-Cell Systems

(20) In view of the successful in vitro conversion of steroids, a biotransformation in whole cells was developed.

(21) Generally, in order to perform the hydroxylation in whole cells, the CYP21 as well as the necessary electron transfer proteins were expressed heterologously in Escherichia coli strain C43(DE3). For expression and following conversion, bi- or tricistronic vectors based on the plasmid pET17b were constructed, which carry the genes for the particular CYP21 and the particular redox system. FIG. 5 shows all constructed vectors. To facilitate correct protein folding, the E. coli chaperones GroEL and GroES were co-expressed on a second vector. The transformed E. coli cells were able to produce the CYP21 enzyme as well as the needed redox partners. After the protein expression, a substrate conversion took place which was started by the addition of the steroid to be hydroxylated (exemplified by medrane) as a substrate.

(22) In particular, E. coli strain C43(DE3) was transformed with vector for whole cell biocatalysis (e.g. p21b_ArET) and the pGro12 which encodes the chaperones GroEL/ES. The culture comprised 200 mL TB medium (+antibiotics ampicillin and kanamycin) in a 2 L Erlenmeyer flask, inoculated with 2 mL seed culture, and was grown at 37? C. Expression was induced at OD 0.5 by addition of 1 mM IPTG, 1 mM ?-aminolevulinic acid, 4 mg/mL arabinose and maintained for 28 h at 27? C. For whole cell biotransformation, cells were harvested by centrifugation and washed with 50 mM potassium phosphate buffer (pH 7.4). Substrate conversion was started with the addition of 400 ?M substrate with resting cells in 25 mL potassium phosphate buffer (50 mM) including 2% glycerol, 1 mM IPTG, 1 mM ?-aminolevulinic acid, 4 mg/mL arabinose in 300 mL buffled flasks for 24 h at a cell density of ca. 24 g/L (wet weight). Samples were taken after, e.g., 24 h and measurement was performed via RP-HPLC after steroid extraction with chloroform.

(23) FIG. 6 shows that the steroid was converted to its 21-hydroxylated derivative and that the appearance of by-products was not observed, in contrast to a chemical synthesis. Time-dependent product formation was studied in whole cells with the six redox systems for each CYP21 isoform to determine not only an endpoint yield but also the velocity of the reaction which is of high interest regarding a biotechnological process (FIG. 7).

(24) Next to the medrane-to-premedrol conversion, both human and bovine CYP21 were able to hydroxylate all tested 3-ketosteroids which are not yet hydroxylated at position 21. In particular, the following steroid conversions could be shown: Medrane to premedrol (non-natural substrate) Deltamedrane to medrol (non-natural substrate) Progesterone to 11-deoxycorticosterone (natural substrate) 170H-progesterone to 11-deoxycortisol (natural substrate) Medroxyprogesterone to 21OH-medroxyprogesterone (non-natural substrate) 5?-dihydroprogesterone to 21OH-(5?-dihydroprogesterone).