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The catalytic effects of perdeuterating the pyridoxal phosphate-dependent enzyme alanine racemase from Geobacillus stearothermophilus are reported. The mass of the heavy perdeuterated form is ~5.5% greater than that of the protiated form, causing kinetic isotope effects (KIEs) of ~1.3 on k(cat) and k(cat)/K(M) for both L- and D-alanine. These values increase when Cα-deuterated alanine is used as the substrate. The heavy-enzyme KIEs of ~3 on k(cat)/K(M) with deuterated substrates are greater than the product of the individual heavy-enzyme and primary substrate KIEs. This breakdown of the rule of the geometric mean is likely due to coupled motion between the protein and the proton-transfer reaction coordinate in the rate-limiting step. These data implicate a direct role for protein vibrational motions in barrier crossing for proton-transfer steps in alanine racemase.

Citation

Michael D Toney, Joan Nieto Castro, Trevor A Addington. Heavy-enzyme kinetic isotope effects on proton transfer in alanine racemase. Journal of the American Chemical Society. 2013 Feb 20;135(7):2509-11

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PMID: 23373756

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