How thiamine diphosphate is activated in enzymes.
The controversial question of how thiamine diphosphate, the biologically active form of vitamin B1, is activated in different enzymes has been addressed. Activation of the coenzyme was studied by measuring thermodynamics and kinetics of deprotonation at the carbon in the 2-position (C2) of thiamine diphosphate in the enzymes pyruvate decarboxylase and transketolase by use of nuclear magnetic resonance spectroscopy, proton/deuterium exchange, coenzyme analogs, and site-specific mutant enzymes. Interaction of a glutamate with the nitrogen in the 1'-position in the pyrimidine ring activated the 4'-amino group to act as an efficient proton acceptor for the C2 proton. The protein component accelerated the deprotonation of the C2 atom by several orders of magnitude, beyond the rate of the overall enzyme reaction. Therefore, the earlier proposed concerted mechanism or stabilization of a C2 carbanion can be excluded.
Kern
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Institut für Biochemie, Martin-Luther Universität Halle-Wittenberg, Kurt-Mothes-Strasse 3, D-06120 Halle, Germany.
Kern
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Neef
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Tittmann
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Killenberg-Jabs
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Wikner
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Schneider
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Hübner
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D000494:Allosteric Regulation; D001665:Binding Sites; D002384:Catalysis; D003903:Deuterium / Q000378:metabolism; D004789:Enzyme Activation; D018698:Glutamic Acid / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D016297:Mutagenesis, Site-Directed; D011522:Protons; D011767:Pyruvate Decarboxylase / Q000737:chemistry / Q000378:metabolism*; D011773:Pyruvates / Q000378:metabolism; D013816:Thermodynamics; D013835:Thiamine Pyrophosphate / Q000737:chemistry / Q000378:metabolism*; D014174:Transketolase / Q000737:chemistry / Q000378:metabolism*
D011522:Protons; D011773:Pyruvates; D018698:Glutamic Acid; D003903:Deuterium; D014174:Transketolase; D011767:Pyruvate Decarboxylase; C009144:pyruvamide; D013835:Thiamine Pyrophosphate