Heavy enzyme isotope effects occur in proteins substituted with ²H-, ¹³C-, and ¹⁵N-enriched amino acids. Mass alterations perturb femtosecond protein motions and have been used to study the linkage between fast motions and transition-state barrier crossing. Heavy enzymes typically show slower rates for their chemical steps. Heavy bacterial methylthioadenosine nucleosidases (MTANs from Helicobactor pylori and Escherichia coli) gave normal isotope effects in steady-state kinetics, with slower rates for the heavy enzymes. However, both enzymes revealed rare inverse isotope effects on their chemical steps, with faster chemical steps in the heavy enzymes. Computational transition-path sampling studies of H. pylori and E. coli MTANs indicated closer enzyme–reactant interactions in the heavy MTANs at times near the transition state, resulting in an improved reaction coordinate geometry. Specific catalytic interactions more favorable for heavy MTANs include improved contacts to the catalytic water nucleophile and to the adenine leaving group. Heavy bacterial MTANs depart from other heavy enzymes as slowed vibrational modes from the heavy isotope substitution caused improved barrier-crossing efficiency. Improved sampling frequency and reactant coordinate distances are highlighted as key factors in MTAN transition-state stabilization.

重酶同位素效应发生在被² H、 ¹³ C 和¹⁵ N 富集氨基酸取代的蛋白质中。质量改变扰乱飞秒蛋白质运动，并已被用来研究快速运动和过渡态势垒穿越之间的联系。重酶的化学步骤通常表现出较慢的速率。重细菌甲硫腺苷核苷酶（来自幽门螺杆菌和大肠杆菌的 MTAN）在稳态动力学中产生正常的同位素效应，但重酶的速率较慢。然而，这两种酶都显示出对其化学步骤的罕见反同位素效应，其中重酶的化学步骤更快。幽门螺杆菌和大肠杆菌MTAN 的计算过渡路径采样研究表明，重 MTAN 中的酶-反应物相互作用有时接近过渡态，从而改善了反应坐标几何形状。对重 MTAN 更有利的特定催化相互作用包括改善与催化水亲核试剂和腺嘌呤离去基团的接触。重细菌 MTAN 与其他重酶不同，因为重同位素取代导致振动模式减慢，从而提高了穿越屏障的效率。改进的采样频率和反应物坐标距离被认为是 MTAN 过渡态稳定的关键因素。