Porcine Carbonyl Reductase

Abstract

Porcine testicular carbonyl reductase (PTCR) belongs to the short chain dehydrogenases/reductases (SDR) superfamily and catalyzes the NADPH-dependent reduction of ketones on steroids and prostaglandins. The enzyme shares nearly 85% sequence identity with the NADPH-dependent human 15-hydroxyprostaglandin dehydrogenase/carbonyl reductase. The tertiary structure of the enzyme at 2.3 Å reveals a fold characteristic of the SDR superfamily that uses a Tyr-Lys-Ser triad as catalytic residues, but exhibits neither the functional homotetramer nor the homodimer that distinguish all SDRs. It is the first known monomeric structure in the SDR superfamily. In PTCR, which is also active as a monomer, a 41-residue insertion immediately before the catalytic Tyr describes an all-helix subdomain that packs against interfacial helices, eliminating the four-helix bundle interface conserved in the superfamily. An additional anti-parallel strand in the PTCR structure also blocks the other strand-mediated interface. These novel structural features provide the basis for the scaffolding of one catalytic site within a single molecule of the enzyme.1HU4 Porcine testicular carbonyl reductase (PTCR) belongs to the short chain dehydrogenases/reductases (SDR) superfamily and catalyzes the NADPH-dependent reduction of ketones on steroids and prostaglandins. The enzyme shares nearly 85% sequence identity with the NADPH-dependent human 15-hydroxyprostaglandin dehydrogenase/carbonyl reductase. The tertiary structure of the enzyme at 2.3 Å reveals a fold characteristic of the SDR superfamily that uses a Tyr-Lys-Ser triad as catalytic residues, but exhibits neither the functional homotetramer nor the homodimer that distinguish all SDRs. It is the first known monomeric structure in the SDR superfamily. In PTCR, which is also active as a monomer, a 41-residue insertion immediately before the catalytic Tyr describes an all-helix subdomain that packs against interfacial helices, eliminating the four-helix bundle interface conserved in the superfamily. An additional anti-parallel strand in the PTCR structure also blocks the other strand-mediated interface. These novel structural features provide the basis for the scaffolding of one catalytic site within a single molecule of the enzyme.1HU4 short chain dehydrogenases/ reductases porcine testicular carbonyl reductase 20β-hydroxysteroid dehydrogenase di-μ-iodobis(ethylenediamine)diplatinum(II) nitrate potassium chloroplatinate carbonyl reductase 4-morpholineethanesulfonic acid The short chain dehydrogenases/reductases (SDR)1 catalyze critical steps of activation and inactivation of steroids, vitamins, prostaglandins, and other bioactive molecules by oxidation and reduction of hydroxyl and carbonyl groups, respectively (1Jörnvall H. Persson B. Krook M. Atrian S. Gonzalez-Duarte R. Jeffery J. Ghosh D. Biochemistry. 1995; 34: 6003-6013Crossref PubMed Scopus (1159) Google Scholar). A member of the SDR superfamily, the multifunctional enzyme porcine testicular carbonyl reductase (PTCR)/3α/β, 20β-hydroxysteroid dehydrogenase (20β-HSD), catalyzes the NADPH-dependent reduction of ketones on androgens, progestins, and prostaglandins, as well as aldehydes and ketones on a large number of xenobiotics (2Tanaka M. Ohno S. Adachi S. Nakajin S. Shinoda M. Nagahama Y. J. Biol. Chem. 1992; 267: 13451-13455Abstract Full Text PDF PubMed Google Scholar, 3Nakajin S. Tamura F. Takase N. Toyoshima S. Biol. & Pharm. Bull. 1997; 20: 1215-1218Crossref PubMed Scopus (18) Google Scholar). The high level of 20β-HSD activity in the enzyme is demonstrated by the reduction of 20-carbonyl groups of C21-steroids, such as conversion of 17α-hydroxyprogesterone to 17α,20β-dihydroxy-4-pregnen-3-one, which is present in pig testes during the neonatal stage (4Nakajin S. Ohno S. Takahashi M. Nishimura K. Shinoda M. Chem. Pharm. Bull. 1987; 35: 2490-2494Crossref PubMed Scopus (23) Google Scholar, 5Nakajin S. Ohno S. Shinoda M. J. Biochem. ( Tokyo ). 1988; 104: 565-569Crossref PubMed Scopus (33) Google Scholar). Purified PTCR also shows vigorous 3α- and 3β-HSD activities with 5α-androstan-17β-ol-3-one (5α-dihydrotestosterone) as a substrate (6Ohno S. Nakajin S. Shinoda M.J. J. Steroid Biochem. Mol. Biol. 1991; 38: 787-794Crossref PubMed Scopus (24) Google Scholar). PTCR is highly homologous to NADP(H)-dependent human and rat CRs, except for its 13 additional amino acid residues at the C terminus (2Tanaka M. Ohno S. Adachi S. Nakajin S. Shinoda M. Nagahama Y. J. Biol. Chem. 1992; 267: 13451-13455Abstract Full Text PDF PubMed Google Scholar). The sequence identity of PTCR with the human CR, also known as 15-hydroxyprostaglandin dehydrogenase/9-ketoprostaglandin reductase, is about 85% (2Tanaka M. Ohno S. Adachi S. Nakajin S. Shinoda M. Nagahama Y. J. Biol. Chem. 1992; 267: 13451-13455Abstract Full Text PDF PubMed Google Scholar). Alignment of amino acid sequences of PTCR and human CR with those of other well known members of SDR superfamily, such as bacterial 3α,20β-HSD (7Ghosh D. Weeks C.M. Grochulski P. Duax W.L. Erman M. Rimsay R.L. Orr J.C. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 10064-10068Crossref PubMed Scopus (231) Google Scholar, 8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar), human 17β-HSD type 1 (9Ghosh D. Pletnev V.Z. Zhu D.-W. Wawrzak Z. Duax W.L. Pangborn W. Labrie F. Lin S.-X. Structure. 1995; 3: 503-513Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar), human 11β-HSD (10Lakshmi V. Monder C. Endocrinology. 1988; 123: 2390-2398Crossref PubMed Scopus (319) Google Scholar), and Drosophila alcohol dehydrogenase (11Villarroya A. Juan E. Egestad B. Jörnvall H. Eur. J. Biochem. 1989; 180: 191-197Crossref PubMed Scopus (35) Google Scholar), suggests that these CRs have a 41-residue insertion at a strategic location, before the conserved Tyr-X-X-X-Lys motif. Nearly all SDRs are known to utilize a Tyr-Lys-Ser triad as catalytic residues (1Jörnvall H. Persson B. Krook M. Atrian S. Gonzalez-Duarte R. Jeffery J. Ghosh D. Biochemistry. 1995; 34: 6003-6013Crossref PubMed Scopus (1159) Google Scholar). The Tyr hydroxyl has been proposed to be the proton donor in an electrophilic attack on the substrate carbonyl in a reduction reaction (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar). The other distinguishing feature of all SDRs is that the functional units are either homotetramers or homodimers (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar, 9Ghosh D. Pletnev V.Z. Zhu D.-W. Wawrzak Z. Duax W.L. Pangborn W. Labrie F. Lin S.-X. Structure. 1995; 3: 503-513Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar, 12Varughese K.I. Skinner M.M. Whiteley J.M. Matthews D.A. Xuong N.H. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 6080-6084Crossref PubMed Scopus (149) Google Scholar, 13Tanaka N. Nonaka T. Nakanishi M. Deyashiki Y. Hara A. Mitsui Y. Structure. 1996; 4: 33-45Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar, 14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar, 15Benach J. Atrian S. Gonzalez-Duarte R. Ladenstein R. J. Mol. Biol. 1998; 282: 383-399Crossref PubMed Scopus (95) Google Scholar). Surprisingly, the active units of both PTCR and human CR were reported to be monomeric (5Nakajin S. Ohno S. Shinoda M. J. Biochem. ( Tokyo ). 1988; 104: 565-569Crossref PubMed Scopus (33) Google Scholar, 16Wermuth B. J. Biol. Chem. 1981; 256: 1206-1213Abstract Full Text PDF PubMed Google Scholar). Although more than half of the 15 or so crystal structures of SDRs to date exhibit the same tetrameric quaternary structure, only the four-helix bundle or the Q axis interface that involves the largest surface area of association (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar) is conserved among SDRs. The other surface of close association of monomers in tetrameric SDRs is the so-called P axis interface involving an anti-parallel pair of strands and a pair of helices (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar) that has been observed only once in a dimeric SDR (17Grimm C. Maser E. Möbus E. Klebe G. Reuter K. Ficner R. J. Biol. Chem. 2000; 275: 41333-41339Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). The interfacial four-helix bundle that borders two catalytic sites and buries hydrophobic surfaces of helices is predicted to be important for the integrity of the active site clefts (18Puranen T. Poutanen M. Ghosh D. Vihko P. Vihko R. Mol. Endocrinol. 1997; 11: 77-86PubMed Google Scholar, 19Ghosh D. Vihko P. Chem. Biol. Interact. 2001; 130: 639-652Google Scholar). Here we present the first crystal structure of a monomeric SDR and explain how the functional determinants of oligomeric SDRs could be retained within a monomer of PTCR having the basic SDR fold. The enzyme was purified as described (5Nakajin S. Ohno S. Shinoda M. J. Biochem. ( Tokyo ). 1988; 104: 565-569Crossref PubMed Scopus (33) Google Scholar). Crystallization and data collection details have also been published (20Ghosh D. Erman M. Pangborn W. Duax W.L. Nakajin S. Ohno S. Shinoda M. J. Steroid Biochem. Mol. Biol. 1993; 46: 103-104Crossref PubMed Scopus (7) Google Scholar). Briefly, the enzyme was crystallized from a solution of 36–37% saturated ammonium sulfate in 10 mm MES buffer, pH 6.0. Crystals of diffraction size were grown by macroseeding. The space group is P43212 and the unit cell parameters area = b = 58.53 Å and c= 165.64 Å with one molecule (288 amino acid residues) in the asymmetric unit. Data collection was carried out on a Rigaku R axis II image plate detector at ambient temperature and processed with the software package DENZO (21Otwinowski Z. Minor W. Methods Enzymol. 1997; 276: 307-326Crossref PubMed Scopus (38572) Google Scholar). Data from three crystals between resolution range 55.2 and 2.30 Å were merged to yield a data set of 10792 unique reflections (F > 0) with an average of about three observations per reflection. The overallRmerge was 0.066 on the intensities. The data were 80% complete overall and 52% complete in the resolution shell between 2.50 and 2.30 Å. The structure was solved by the single isomorphous replacement and anomalous scattering method, combined with phases from a molecular replacement solution. Two platinum compounds, potassium chloroplatinate (PtCl4) and di-μ-iodobis(ethylenediamine)diplatinum(II) nitrate (PIP), were used to prepare heavy atom derivative crystals, both yielding the same major platinum-binding site. A total of 4351 reflections to 3.00 Å was phased using the PIP derivative, and 3564 reflections to 3.20 Å were phased using the PtCl4 derivative. The latter data set also contained anomalous signal for 1660 reflections to 3.40 Å. The centricR value and the phasing power for the PIP and PtCl4 derivatives were 0.56 and 1.64 and 0.63 and 1.41, respectively. The phasing power for the anomalous data was 1.92. The software package PHASES (22Furey W. Swaminathan S. Abstracts of the American Crystallographic Association Meeting. New Orleans, LA1990: 73Google Scholar) was used for calculating and combining phases. The molecular replacement solution was obtained using a search model, which was a superposition of equally weighted structures of 3α,20β-HSD (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar), 17β-HSD1 (9Ghosh D. Pletnev V.Z. Zhu D.-W. Wawrzak Z. Duax W.L. Pangborn W. Labrie F. Lin S.-X. Structure. 1995; 3: 503-513Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar), mouse lung carbonyl reductase (13Tanaka N. Nonaka T. Nakanishi M. Deyashiki Y. Hara A. Mitsui Y. Structure. 1996; 4: 33-45Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar), and 7α-HSD (14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar). A large number of rotation search solutions were computed using compact (without loop regions) and extended (all loops through the catalytic triad present) search models. The optimal solution was obtained with the CNS program suite (23Brünger A.T. Adams P.D. Clore G.M. DeLano W.L. Gros P. Grosse-Kunstleve R.W. Jiang J. M. T. D. 1998; PubMed Scopus Google Scholar) and the extended search with data between and against an The search was in both P43212 and space from the solutions were computed and used to a with the PIP derivative and data to 3.00 Å. The solution in the space group P43212 the largest at the platinum-binding having a about three the The heavy atom and the of were by from the were combined with single isomorphous replacement and anomalous scattering phases using the PHASES In reflections were phased to 3.00 with a combined of of a of and an value of was carried out on a using a of weighted phased with the combined and the software J. Mol. 1988; Google Scholar). of amino were in the which also the The was to of and using A. T. New Scholar) and CNS (23Brünger A.T. Adams P.D. Clore G.M. DeLano W.L. Gros P. Grosse-Kunstleve R.W. Jiang J. M. T. D. 1998; PubMed Scopus Google Scholar) The resolution was extended to 2.30 Å. The was during the latter of the with phases were computed to of for a residues in highly of the such as and were in with A sulfate and well molecules were in the three of and A of is in The of the molecular was with J.M. J. 1993; Google Scholar), CNS (23Brünger A.T. Adams P.D. Clore G.M. DeLano W.L. Gros P. Grosse-Kunstleve R.W. Jiang J. M. T. D. 1998; PubMed Scopus Google Scholar), and Biol. 1994; PubMed Scopus Google Scholar) The were with J. Mol. 1993; 11: PubMed Scopus Google Scholar). The have been with the Data of of used > (F > (F > from in in and in a amino acid residues of one chain of PTCR the were from acid residues are with the at in the sequence (2Tanaka M. Ohno S. Adachi S. Nakajin S. Shinoda M. Nagahama Y. J. Biol. Chem. 1992; 267: 13451-13455Abstract Full Text PDF PubMed Google Scholar). A of the tertiary structure of PTCR is in A of the structure is in 1 The basic SDR fold a to by three helices on and The to is a with and the to the with the of the the to in to within the quaternary association and substrate The two helices and the interfacial four-helix of dimeric and tetrameric The in 1 a is the of the PTCR the active site by the and catalytic residues and in a and the basic SDR fold described is well in PTCR, except for two important the 41-residue insertion before describes helices and The first of with the immediately the catalytic in present in nearly all SDRs. all-helix subdomain from the active site the interface of other SDRs. a the describes an additional strand that is anti-parallel to The 13 residues an extended structure that the active site of a The group a with the chain of the A anti-parallel strand is present only in one other member of the SDR reductase K.I. Skinner M.M. Whiteley J.M. Matthews D.A. Xuong N.H. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 6080-6084Crossref PubMed Scopus (149) Google Scholar). 1 is a superposition of monomers of crystal structures of all to the SDR superfamily human 7α-HSD (14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar), and The in is the same as in 1 3α,20β-HSD and 7α-HSD have 17β-HSD1 is The structural among monomers of all members of the SDR is well by The of the fold and the catalytic triad among these the 41-residue insertion in PTCR, is two helices and from and 17β-HSD1 that the four-helix bundle interface well with the from PTCR, which are in The other important from superposition is the of the structures of the by the of which the of the of the active that are critical to the of and to the of molecular by the enzyme D. Vihko P. Chem. Biol. Interact. 2001; 130: 639-652Google Scholar, Erman M. T. Vihko P. Ghosh D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). Although is well for 17β-HSD1 having additional structural from an insertion between and and from an extended C terminus that the catalytic residues from of 3α,20β-HSD and 7α-HSD are present in the In a of the chain from the loop between and to and residues to the active site (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar). in residues to between and and residues to the active site (14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar). the structures in 3α,20β-HSD and 7α-HSD from that of These in the of active sites explain 17β-HSD1 has high for as a the other two are known to be all three the so-called substrate between and which on the substrate its the active site additional (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar, 14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar, 19Ghosh D. Vihko P. Chem. Biol. Interact. 2001; 130: 639-652Google Scholar, Erman M. T. Vihko P. Ghosh D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). The to the active site in PTCR is more than other is a of a between and and to residues and in the loop is in only residues to from the are in the of the active site. The number of residues that the is only 15 in PTCR, as to in in and in The of the is with the observed in substrate of The chain in human CR is to M. Ghosh D. R. M. Jörnvall H. Proc. Natl. Acad. Sci. U. S. A. 1993; PubMed Scopus Google Scholar). chain in PTCR is in a highly and its is the atom to The Q axis interface involving the four-helix bundle and the P axis interface involving anti-parallel association of strands and helices in the tetrameric 3α,20β-HSD are in a and The four-helix conserved in dimeric SDRs such as 17β-HSD1 in of the hydrophobic surfaces that are at (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar). to the Q axis interface by in and monomers (18Puranen T. Poutanen M. Ghosh D. Vihko P. Vihko R. Mol. Endocrinol. 1997; 11: 77-86PubMed Google Scholar). the four-helix bundle is critical to of the bundle with the integrity of the two active sites as in a The P axis interface hydrophobic (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar) and is critical to the structural integrity of the active site. The all-helix subdomain of the acid insertion to and and and the chain from the of the strand such that of its are from The and a half the in the and packs of the of as in and the of helices and in and the interface and as in a of the area in the interface and of residues on these helices in PTCR the surface more than in oligomeric as in from 3α,20β-HSD to PTCR are as to to to to and to helices of the insertion are hydrophobic surfaces the hydrophobic surfaces of and and surfaces are to the as for in the hydrophobic surface of which packs against in close to the both from The atom of is at a of Å from of which is Å from of is for the high of platinum and to the that in the only isomorphous derivatives for the Although was in the a molecule of was to the The is in the extended as observed for all other to SDRs (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar, 12Varughese K.I. Skinner M.M. Whiteley J.M. Matthews D.A. Xuong N.H. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 6080-6084Crossref PubMed Scopus (149) Google Scholar, 13Tanaka N. Nonaka T. Nakanishi M. Deyashiki Y. Hara A. Mitsui Y. Structure. 1996; 4: 33-45Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar, 14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar, 15Benach J. Atrian S. Gonzalez-Duarte R. Ladenstein R. J. Mol. Biol. 1998; 282: 383-399Crossref PubMed Scopus (95) Google Scholar, Erman M. T. Vihko P. Ghosh D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). The between of the and of the a used to the of of is well within the range for SDRs. The is in with the the catalytic of all SDRs. The molecule and its are in with in the of the was at the and we used parameters of an to its during and between and amino acid residues are in The high of the enzyme for has been pH PTCR has the for as by the of the of of all (6Ohno S. Nakajin S. Shinoda M.J. J. Steroid Biochem. Mol. Biol. 1991; 38: 787-794Crossref PubMed Scopus (24) Google Scholar). In the crystal structure, we that the is by three and one chain and a of these with the either through or as in The chain carbonyl also a proton from the of the In to the of and residues the of before (13Tanaka N. Nonaka T. Nakanishi M. Deyashiki Y. Hara A. Mitsui Y. Structure. 1996; 4: 33-45Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar, Erman M. T. Vihko P. Ghosh D. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar), of the chain with and groups an important in The of the group of the is to the in 3α,20β-HSD (8Ghosh D. Wawrzak Z. Weeks C.M. Duax W.L. Erman M. Structure. 1994; 2: 629-640Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar) and the in 7α-HSD (14Tanaka N. Nonaka T. Tanabe T. Yoshimoto T. Tsuru D. Mitsui Y. Biochemistry. 1996; 35: 7715-7730Crossref PubMed Scopus (218) Google Scholar). The group of the with the chain of the chain a with the a group from is also present in the as has been observed in other SDR structures in 3α,20β-HSD and in Two molecules are to the and and the chain carbonyl group of have with the and of the as in in the is a short between the chain carbonyl of of PTCR, conserved in all and other and of the and the of carbonyl with to the have been proposed to have an important in the from the to the substrate D. Vihko P. Chem. Biol. Interact. 2001; 130: 639-652Google Scholar). The among monomers of PTCR is the of a by the of two loops between and to about a rotation of the of the active site the loops active site and the chain from one loop two to the catalytic residues and and of the other as in of the hydroxyl group of the catalytic Tyr with a acid group was observed in the structure of the of 3α,20β-HSD D. Erman M. Wawrzak Z. Duax W.L. Pangborn W. Structure. 1994; 2: Full Text Full Text PDF PubMed Scopus Google Scholar), in which the chain of the a with the The chain of a with from a The rotation axis through the of two are Å and with the chain of of The chain against the and and chain against the The of loop is in PTCR than in other SDRs The of the of the loops in and 7α-HSD is by the of a in in and in which is in The loop an extended in The between and the in in and in 7α-HSD are and respectively. It is in of the loop from the of the chain with catalytic residues or to The other of is the of a between the of one molecule and the chain of the as in is the same chain that is in a with the group of the the extended loop described the of the monomer, the among with with and with and with the two extended and of the PTCR molecule are in the crystal by with the same of a between two molecules only be present in the as the functional unit of PTCR was to be a monomer in solution from the data (5Nakajin S. Ohno S. Shinoda M. J. Biochem. ( Tokyo ). 1988; 104: 565-569Crossref PubMed Scopus (33) Google Scholar). structures of oligomeric SDRs demonstrated how of could be to functional with active or is for be an of The PTCR structure how of the by and insertion the of among to scaffolding of one functional active site within an It is that monomeric SDRs PTCR have in the than oligomeric