A longstanding conundrum in Treponema pallidum biology concerns how the spirochete generates sufficient energy to fulfill its complex pathogenesis processes during human syphilitic infection. For decades, it has been assumed that the bacterium relies solely on glucose catabolism (via glycolysis) for generation of its ATP. However, the organism’s robust motility, believed to be essential for human tissue invasion and dissemination, would require abundant ATP likely not provided by the parsimony of glycolysis. As such, additional ATP generation, either via a chemiosmotic gradient, substrate-level phosphorylation, or both, likely exists in T. pallidum. Along these lines, we have hypothesized that T. pallidum exploits an acetogenic energy conservation pathway that relies on the redox chemistry of flavins. Central to this hypothesis is the apparent existence in T. pallidum of an acetogenic pathway for the conversion of d-lactate to acetate. Herein we have characterized the structural, biophysical, and biochemical properties of the first enzyme (d-lactate dehydrogenase [d-LDH]; TP0037) predicted in this pathway. Binding and enzymatic studies showed that recombinant TP0037 consumed d-lactate and NAD⁺ to produce pyruvate and NADH. The crystal structure of TP0037 revealed a fold similar to that of other d-acid dehydrogenases; residues in the cofactor-binding and active sites were homologous to those of other known d-LDHs. The crystal structure and solution biophysical experiments revealed the protein’s propensity to dimerize, akin to other d-LDHs. This study is the first to elucidate the enzymatic properties of T. pallidum’s d-LDH, thereby providing new compelling evidence for a flavin-dependent acetogenic energy conservation (ATP-generating) pathway in T. pallidum.

中文翻译：

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TP0037（一种 d-乳酸脱氢酶）的生物物理和生化表征支持梅毒螺旋体中的产乙酸能量守恒途径。

梅毒螺旋体生物学中的一个长期难题是螺旋体如何在人类梅毒感染期间产生足够的能量来完成其复杂的发病过程。几十年来，人们一直认为细菌仅依赖葡萄糖分解代谢（通过糖酵解）来产生 ATP。然而，生物体的强劲运动被认为对于人体组织的侵袭和传播至关重要，因此需要丰富的 ATP，而糖酵解的简约性可能无法提供这些 ATP。因此，梅毒螺旋体中可能存在通过化学渗透梯度和/或底物水平磷酸化产生额外的 ATP 。沿着这些思路，我们假设梅毒螺旋体利用了一种依赖于黄素氧化还原化学的产乙酸能量守恒途径。这一假说的核心是梅毒螺旋体中明显存在将d-乳酸转化为乙酸的产乙酸途径。在此，我们表征了该途径中预测的第一种酶（ d-乳酸脱氢酶[ d -LDH]；TP0037）的结构、生物物理和生化特性。结合和酶学研究表明，重组 TP0037 消耗d-乳酸和 NAD ⁺产生丙酮酸和 NADH。TP0037的晶体结构显示出与其他d-酸脱氢酶相似的折叠；辅因子结合和活性位点中的残基与其他已知的d -LDH 的残基同源。晶体结构和溶液生物物理实验揭示了该蛋白质的二聚倾向，类似于其他d -LDH。这项研究首次阐明了梅毒螺旋体的d -LDH 酶特性，从而为梅毒螺旋体中黄素依赖性产乙酸能量守恒（ATP 生成）途径提供了新的令人信服的证据。