Ketosteroid Isomerase: Difference between revisions
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===Dimer Interface=== | ===Dimer Interface=== | ||
The biologically active unit of KSI is a 2-fold symmetric dimer in which the two chains are packed together via hydrophobic and electrostatic interactions between the "back faces" of the β-sheets. The curved β-sheets on each of the monomers expose convex faces to each other, forming well-defined interactions. In their NMR structure of KSI, Massiah et al. identified interchain hydrophobic interactions (A), as well, a number of polar residues (B) located within the dimer interface. Although they were unable to resolve interchain hydrogen-bonding interactions, they were able to resolve an intrachain hydrogen bond between His100 and Glu77 (C). The apparent pKa of His100 is 4.3, and the authors speculate that this unusually low pKa may be the result of electrostatic effects from nearby cationic residues (Arg102 and Arg113). | The biologically active unit of KSI is a 2-fold symmetric dimer in which the two chains are packed together via hydrophobic and electrostatic interactions between the "back faces" of the β-sheets. The curved β-sheets on each of the monomers expose convex faces to each other, forming well-defined interactions. In their NMR structure of KSI, Massiah et al. identified interchain hydrophobic interactions (A), as well, a number of polar residues (B) located within the dimer interface. Although they were unable to resolve interchain hydrogen-bonding interactions, they were able to resolve an intrachain hydrogen bond between His100 and Glu77 (C). The apparent pKa of His100 is 4.3, and the authors speculate that this unusually low pKa may be the result of electrostatic effects from nearby cationic residues (Arg102 and Arg113). | ||
[[Image:Massiah.png|center]] | [[Image:Massiah.png|center]] | ||
In their X-ray crystal structure of KSI, Kim et al. identified bound water molecules within the dimer interface which may mediate hydrogen-bonds between Thr68, Arg72, and Asp96. The water mediated hydrogen bond between Thr68 and Arg72 is illustrated below. Kim et al. also identified interchain hydrogen bonds between the backbone carbonyl oxygens of Val71, Ala73, and Val97. | |||
[[Image:Kim1.png|left]] [[Image:Kim2.png|right]] | |||
===Hydrophobic Active Site=== | ===Hydrophobic Active Site=== | ||
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Although the <scene name='User:Laura_M._Haynes/Sandbox_1/Active_site_1isk/1'>active site</scene> 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 forming hydrogen bonds with the 3-position carbonyl of the steroid and form an active site oxyanion hole.<ref name="Pollack" />,<ref name="Sigala2008">PMID:18808119 </ref> Additionally, the active site contains an aspartate residue at position 38 that is participates in the catalytic activity of KSI.<ref name="Pollack" /> [[Image:cis.png|right]] | Although the <scene name='User:Laura_M._Haynes/Sandbox_1/Active_site_1isk/1'>active site</scene> 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 forming hydrogen bonds with the 3-position carbonyl of the steroid and form an active site oxyanion hole.<ref name="Pollack" />,<ref name="Sigala2008">PMID:18808119 </ref> Additionally, the active site contains an aspartate residue at position 38 that is participates in the catalytic activity of KSI.<ref name="Pollack" /> [[Image:cis.png|right]] | ||
===Cis-Peptide Bond=== | ===Cis-Peptide Bond=== | ||
