Ketosteroid Isomerase
From Proteopedia
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==Enzyme Mechanism== | ==Enzyme Mechanism== | ||
| - | The hydrophobic active site of | + | 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.<ref name="Pollack" /> 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.<ref name="Pollack" />,<ref name="Ha" /> Structural and kinetic studies suggest that Asp<sup>38</sup> (numbering is that of the TI varient of KSI) serves as a general base in this reaction. |
| + | 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.<ref name="Pollack" /> | ||
==Related Proteins== | ==Related Proteins== | ||
Revision as of 17:53, 3 April 2010
Contents |
Ketosteroid Isomerase
Introduction
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Ketosteroid isomerase (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 It is one of the most efficient known enzymes with an essentially diffusion limited rate of catalysis.[2]
An NMR solution phase structure of KSI was solved in 1997 by Wu et al.[4] allowing greater insight into the mechanism of this intriguing enzyme.
Structure
Ketosteroid isomerase exits as a 28 kDa homodimeric protein, in which the two dimers related to each other via hydrophobic and electrostatic interactions.[4] Each dimer 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],[5] 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]
Enzyme 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. 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]
Related Proteins
Available Structures
References
- ↑ 1.0 1.1 1.2 1.3 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
- ↑ 2.0 2.1 Smith SB, Richards JW, Benisek WF. The purification and characterization of delta 5-3-ketosteroid isomerase from Pseudomonas putida, a cysteine-containing isomerase. J Biol Chem. 1980 Apr 10;255(7):2678-84. PMID:7358699
- ↑ 3.0 3.1 3.2 3.3 3.4 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
- ↑ 4.0 4.1 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
- ↑ 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
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