Ketosteroid Isomerase

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Revision as of 14:21, 30 April 2010

Ketosteroid Isomerase

Introduction

Template:STRUCTURE 1isk (KSI, EC#5.3.3.1) is an enzyme that catalyzes the isomerization of 3-oxo-Δ⁵ ketosteroids to their hormonally active Δ⁴-conjugated isomers, as illustrated below.^[1], ^[2]

This reaction is essential in the biosynthesis of steroids in mammals where KSI is a membrane-bound complex.^[3] In bacteria, however, KSI exists as a soluble protein is involves in catabolism of steroids.^[3] It was first isolated in and has been extensively studied in Commamonas tetosteroni (TI), a bacteria that is capable of growth with testosterone as its sole carbon source.^[4] Structural and kinetic studies of this and its homolog from Pseudomonas putida with which it shares 34% sequence and near identical structural homology.^[1],^[3] It is one of the most efficient known enzymes with an essentially diffusion limited rate of catalysis.^[2],^[5] It is capable of increasing the catalytic rate by eleven orders of magnitude.^[6]

Structure

Ketosteroid isomerase exits as a 28 kDa homodimeric protein, in which the two dimers related to each other via hydrophobic and electrostatic interactions.^[5] Each monomer consists of a curved and three . These secondary structures define a conical closed barrel geometry, with one open and one closed end, and create a deep pocket in which the active site resides.^[3],^[7] This unique geometry is shared by several other proteins (scytalone dehydratase, nuclear transport factor 2, and naphthalene 1,2-dioxygenase), however, these molecules do not share functional or sequence homology. It is speculated that this unique protein structure may enable better binding of hydrophobic substrates such as steroids.^[3]

Although the of KSI is notably hydrophobic, it contains several hydrophilic residues believed to be important to the enzymatic function of the protein. The hydrophobic active site of KSI contains an aspartate residue at position 99 and a tyrosine residue at position 14 (according to the numbering for the Commamonas tetosteroni protein, which will be used throughout) that are capable of binding the 3-position carbonyl of the steroid. Additionally, the active site contains an aspartate residue at position 38 that is participates in the catalytic activity of KSI.^[1]

Enzyme Mechanism

General Mechanism

The general mechanism of the proposed reaction of ketosteroid isomerase involves the breaking of a C-H bond adjacent to a carbonyl. This is typically regarded as a difficult reaction due to instability of the intermediate; however it is observed in a number of enzyme-mediated biological reactions.^[1] In line with other biological reactions, the mechanism of KSI involves the abstraction the β-hyrdogen from the 4-position carbon resulting in the formation of an enol intermediate, which is followed by reketonization.^[1],^[3] Structural and kinetic studies suggest that Asp³⁸ (numbering is that of the TI varient of KSI) serves as a general base in this reaction as shown below and abstracts the %beta;-proton from C4 with the syn orbitals of its carboxylate group.^[5] Note the formation of the unstable enolate intermediate.

Tyr¹⁴ and Asp⁹⁹ are believed to participate in hydrogen bonding to the O-3 carbonyl of the substrate steroid and stabilize reaction intermediates. Tyr¹⁴ is also believed to participate in a low barrier hydrogen bond with the 3-postition oxygen of the steroid, thereby facilitating the abstraction of the β-hydrogen at the 4-position. There are two proposed models of this hydrogen bonding. In , Tyr¹⁴ and Asp⁹⁹ are both bound to the 3-position oxygen, whereas, in , they form a hydrogen bonding network. These are as a single monomer in complex with the intermediate analog equilenin.^[8]

Model 1 has become the generally accepted scheme through both crystallographic and mutanagenic/kinetic approaches. Mutation of Asp⁹⁹ to alanine and Tyr¹⁴ to phenylalanine resulted in deleterious effects on kinetic parameters, which were additive in nature suggesting that Tyr¹⁴ and Asp⁹⁹ participate equally in hydrogen binding with the oxygen atom. Additionally, the crystal structure of TI analog from Pseudomonas putida supports this conclusion given the .^[1] The general mechanism in the active site of KSI is outlined below.

Low Barrier Hydrogen Bond

Ketosteroid isomerase has become a test enzyme in the debate over the existence of low barrier hydrogen bonds (LBHBs). Nuclear magnetic resonance (NMR) studies of KSI in complex with transition state analogs have revealed the presence of a highly shielded proton characteristic of the formation of a LBHB between Tyr¹⁴ and the O-3 atom of the analogs and suggestive of the formation of a bridging hydrogen with short bonds.^[3],^[9] Additionally, NMR fractionation studies with deuterium substitution are strongly suggestive of the hight strength of this bond as deuterium is retained preferentially in this position.^[10]. identified Myobacterium tuberculosis Rv0760c as a structural homolog of KSI. Although these proteins share similar structural motifs there carry out a diverse array of functions.^[11]

Available Structures

• 1isk-First solution phase structure NMR structure of KSI from Comamonas testosteroni
• 1opy
• 1buq
• 1qjg-KSI from Pseudomoas testosteroni in complex with equilenin
• 1c7h
• 1e3r
• 1e3v
• 1e97
• 1ea2
• 1k41
• 1gs3
• 1ogx
• 1oh0-KSI from Pseudomonas putida in complex with equilenin
• 1cqs
• 1vzz
• 1w01
• 1w02
• 1oho-KSI Y16F/D40N mutant from Pseudomonas putida in complex with equilenin
• 1w6y
• 1w0o
• 2pzv
• 2inx
• 3cpo
• 3ex9
• 3fzw
• 3ipt
• 3m8c

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5} Pollack RM. Enzymatic mechanisms for catalysis of enolization: ketosteroid isomerase. Bioorg Chem. 2004 Oct;32(5):341-53. PMID:15381400 doi:10.1016/j.bioorg.2004.06.005
2. ↑ ^{2.0 2.1} TALALAY P, WANG VS. Enzymic isomerization of delta5-3-ketosteroids. Biochim Biophys Acta. 1955 Oct;18(2):300-1. PMID:13276386
3. ↑ ^{3.0 3.1 3.2 3.3 3.4 3.5 3.6} Ha NC, Choi G, Choi KY, Oh BH. Structure and enzymology of Delta5-3-ketosteroid isomerase. Curr Opin Struct Biol. 2001 Dec;11(6):674-8. PMID:11751047
4. ↑ Ha NC, Choi G, Choi KY, Oh BH. Structure and enzymology of Delta5-3-ketosteroid isomerase. Curr Opin Struct Biol. 2001 Dec;11(6):674-8. PMID:11751047
5. ↑ ^{5.0 5.1 5.2} Wu ZR, Ebrahimian S, Zawrotny ME, Thornburg LD, Perez-Alvarado GC, Brothers P, Pollack RM, Summers MF. Solution structure of 3-oxo-delta5-steroid isomerase. Science. 1997 Apr 18;276(5311):415-8. PMID:9103200
6. ↑ Murzin AG. How far divergent evolution goes in proteins. Curr Opin Struct Biol. 1998 Jun;8(3):380-7. PMID:9666335
7. ↑ Cho HS, Choi G, Choi KY, Oh BH. Crystal structure and enzyme mechanism of Delta 5-3-ketosteroid isomerase from Pseudomonas testosteroni. Biochemistry. 1998 Jun 9;37(23):8325-30. PMID:9622484 doi:10.1021/bi9801614
8. ↑ Cho HS, Ha NC, Choi G, Kim HJ, Lee D, Oh KS, Kim KS, Lee W, Choi KY, Oh BH. Crystal structure of delta(5)-3-ketosteroid isomerase from Pseudomonas testosteroni in complex with equilenin settles the correct hydrogen bonding scheme for transition state stabilization. J Biol Chem. 1999 Nov 12;274(46):32863-8. PMID:10551849
9. ↑ Zhao Q, Abeygunawardana C, Talalay P, Mildvan AS. NMR evidence for the participation of a low-barrier hydrogen bond in the mechanism of delta 5-3-ketosteroid isomerase. Proc Natl Acad Sci U S A. 1996 Aug 6;93(16):8220-4. PMID:8710850
10. ↑ The energy of this bond along with that of the normal hydrogen bond from Asp⁹⁹ most likely contribute to the energy needed to support proton abstraction from the C-4 position.<ref></ref>

    Contents 1 =Implications of Hydrophobic Active Site 2 Related Proteins 2.1 Functionally Related Proteins 2.2 Structural Homologs

    =Implications of Hydrophobic Active Site

    In combination with the above mentioned hydrogen bond interactions between the substrate and the active site, the highly hydrophobic active site also serves to stabilize the catalytic efficiency of KSI because it is in a preorganized state compared to the of the reaction carried out in solvent where acetate anion is used to mimic the functionality of Asp³⁸ and has a similar pKaof 4.6. To prevent divergence of Asp³⁸ from this normal pKa during initial substrate binding, it is hydrogen bonded to two or three water molecules which are excluded from the active site later in the enzymatic reaction. Maintenance of this normal pKa is important because protonation of Asp³⁸ would no longer permit it to carry out hydrogen abstraction.<ref></ref>

    Related Proteins

    Functionally Related Proteins

    Ketosteroid isomerase is one of a large number of enzymes that catalyze the cleavage of a C-H bond. Among these are mandelate racemace, triosephosphate isomerase, citrate synthase, and 4-oxalocrotonate tautomerase.<ref></ref>

    Structural Homologs

    As noted above, scytalone dehydratase, nuclear transport factor 2 (NTF2), and naphthalene 1,2-dioxygenase share similar structural motifs to ketosteroid isomerase, which facilitate binding of hydrophobic substrates.<ref></ref>,<ref></ref> It has also been postulated that bile acid 7α-dehydratase is also a member of this family of protein and that it shares functional similarity with KSI.<ref></ref> In a recent paper, Cherney et al<ref>PMID:18589008 </li> <li id="cite_note-Cherney">[[#cite_ref-Cherney_0|↑]] <strong class="error">Cite error: Invalid <code>&lt;ref&gt;</code> tag;

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