DOPA decarboxylase: Difference between revisions
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===Flexible Loop=== | ===Flexible Loop=== | ||
In all three crystal structures of DOPA decarboxylase solved to date, residues 328-339 are invisible in the electron density map. This is because these amino acids form a short mobile loop that is believed to be important to the catalytic mechanism of the enzyme <ref name="ishii">PMID:10082378 </ref>. Although the highly conserved'''Tyr332 residue''' was found to be essential for catalytic activity, its role was unknown prior to solving the crystal structure. During catalysis, this loop is proposed to lose its flexibility and extend toward the active site, both occluding the active site from solvent during catalysis and possibly even taking part in the catalytic mechanism. Based on the catalytic role of Tyr in other PLP-dependent enzymes, Tyr332 could act as a proton donor for the quinonoid Cα. For example, the mobile loop is found in other PLP-depended enzymes, such as glutamate 1-semialdehyde aminotransferase. | In all three crystal structures of DOPA decarboxylase solved to date, residues 328-339 are invisible in the electron density map. This is because these amino acids form a short mobile loop that is believed to be important to the catalytic mechanism of the enzyme <ref name="ishii">PMID:10082378 </ref>. Although the highly conserved''' Tyr332 residue''' was found to be essential for catalytic activity, its role was unknown prior to solving the crystal structure. During catalysis, this loop is proposed to lose its flexibility and extend toward the active site, both occluding the active site from solvent during catalysis and possibly even taking part in the catalytic mechanism. Based on the catalytic role of Tyr in other PLP-dependent enzymes, Tyr332 could act as a proton donor for the quinonoid Cα. For example, the mobile loop is found in other PLP-depended enzymes, such as glutamate 1-semialdehyde aminotransferase. | ||
==Mechanism== | ==Mechanism== | ||
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The mechanism of DDC catalyzed decarboxylation of L-Dopa to Dopamine has been well-studied due to the enzymes role in PD. Shown below is the mechanism of decarboxylation, as determined by several experimental approaches, including site-directed mutagenesis, UV-Vis Spectroscopy, X-Ray Crystallography, Multiple Sequence Alignment, and Stopped-Flow Spectroscopy. [[image:slide1.png|thumb|center|500px|]] Once again, the transimination step (conversion of internal to external aldimine) is common to all PLP-dependent enzymes. The subsequent orientation of the quinonoid intermediate allows for stereospecific decarboxylation of the substrate at the alpha carbon, as predicted by '''Dunathan's stereoelectronic hypothesis''' <ref name="dunathan">PMID:224217 </ref>, in which he proposed that the substrate binds PLP such that the bond made around the Cα that is going to be broken is aligned with the pi orbital system of the cofactor. | The mechanism of DDC catalyzed decarboxylation of L-Dopa to Dopamine has been well-studied due to the enzymes role in PD. Shown below is the mechanism of decarboxylation, as determined by several experimental approaches, including site-directed mutagenesis, UV-Vis Spectroscopy, X-Ray Crystallography, Multiple Sequence Alignment, and Stopped-Flow Spectroscopy. [[image:slide1.png|thumb|center|500px|]] Once again, the transimination step (conversion of internal to external aldimine) is common to all PLP-dependent enzymes. The subsequent orientation of the quinonoid intermediate allows for stereospecific decarboxylation of the substrate at the alpha carbon, as predicted by '''Dunathan's stereoelectronic hypothesis''' <ref name="dunathan">PMID:224217 </ref>, in which he proposed that the substrate binds PLP such that the bond made around the Cα that is going to be broken is aligned with the pi orbital system of the cofactor. | ||
[[image:Dunathan.png|thumb|left|300px|'''Dunathan's Stereoeletronic Hypothesis, 1966''']] This way, the developing p orbital is aligned for maximal overlap with the extended p system, lowering the energy of the transition state and increasing the rate of the reaction. As well, by controlling substrate orientation, the enzyme can distinguish between '''deprotonation''' and '''decarboxylation'''. | [[image:Dunathan.png|thumb|left|300px|'''Dunathan's Stereoeletronic Hypothesis, 1966''']] This way, the developing p orbital is aligned for maximal overlap with the extended p system, lowering the energy of the transition state and increasing the rate of the reaction. As well, by controlling substrate orientation, the enzyme can distinguish between '''deprotonation''' and '''decarboxylation'''. | ||
===Critical Residues=== | |||
====Lys303==== | |||
====Asp271==== | |||
====His192==== | |||
====Arg355==== | |||
==Classification== | ==Classification== | ||
