Mechanistic origins of variation in macromolecular-rate-theory temperature-sensitivity traits ($\Delta C_p^{\ddagger}$, $T_{opt}$, $TS_{max}$) of soil microbial enzymes

Macromolecular rate theory (MMRT) describes the temperature response of enzyme-catalyzed rates using three traits: the heat capacity of activation ($\Delta C_p^{\ddagger}$), the temperature optimum ($T_{opt}$), and the temperature of maximum temperature sensitivity ($TS_{max}$). Alster and colleagues showed in 2016 that these traits vary among seven soil isolates and three extracellular enzymes, with microbial isolate identity explaining most of the variance in $\Delta C_p^{\ddagger}$, but the study could not attribute the observed differences to mechanisms. Subsequent work has reshaped this question without closing it: meta-analytic work has mapped how broadly the traits vary across organisms and processes, model-enzyme studies have established that changes in enzyme rigidity causally alter $\Delta C_p^{\ddagger}$, and geothermal-gradient studies have quantified community-level thermal adaptation rates; meanwhile an alternative chemical-kinetics theory now contests the interpretation of $T_{opt}$ as an enzyme-intrinsic property by showing substrate availability can shift it. The precise open core is the mechanistic attribution of trait variation in soil microbial enzymes among enzyme flexibility, substrate type, genetic variation, and isoenzyme composition, together with the condition dependence (e.g., pH) of the MMRT temperature-response curve and the acclimation, adaptation, and evolutionary constraints governing these traits under climate warming.

Background

Soil microbes decompose organic matter through extracellular enzymes — enzymes secreted outside the cell, such as β-glucosidase, leucine aminopeptidase, and phosphatase, which break down carbon-, nitrogen-, and phosphorus-bearing substrates. The rates of these enzyme-catalyzed reactions increase with temperature but typically decline again at higher temperatures, producing a concave-down curve that the classical Arrhenius equation and the $Q_{10}$ temperature coefficient (the factor by which a rate increases over a 10 °C interval) cannot represent, since both predict a monotonic exponential increase. Macromolecular rate theory (MMRT) extends transition state theory — the framework in which reaction rates are governed by the free energy barrier ($\Delta G^{\ddagger}$) between the ground state and the transition state — by allowing the heat capacity of activation ($\Delta C_p^{\ddagger}$) to be nonzero. Because a negative $\Delta C_p^{\ddagger}$ makes both the enthalpy and entropy of activation temperature-dependent, MMRT naturally produces the observed curved rate-versus-temperature relationship without invoking thermal denaturation; mechanistically, a negative $\Delta C_p^{\ddagger}$ is interpreted as a narrowing of the enzyme's conformational ensemble (reduced flexibility) in the transition state. MMRT yields three measurable summary traits: $\Delta C_p^{\ddagger}$ (the curvature), $T_{opt}$ (the temperature of maximum rate), and $TS_{max}$ (the temperature at which the rate of change of the reaction rate with temperature is greatest, typically within environmentally relevant ranges). Hobbs et al. (2013) established MMRT for purified enzymes, Schipper et al. (2014) showed it fits soil biogeochemical rates better than Arrhenius-type functions, and Arcus et al. (2016) set out the theory. Alster et al. (2016) then measured the temperature responses of three extracellular enzymes from seven soil bacterial isolates (19 usable isolate × enzyme combinations at six temperatures from 4 to 60 °C) and proposed temperature sensitivity, in the MMRT parametrization, as an intrinsic microbial functional trait — a measurable organismal property that could link community composition to ecosystem nutrient cycling. They found $\Delta C_p^{\ddagger}$ differed among isolates for two of the three enzymes (with isolate identity explaining 70.9% of the variance in $\Delta C_p^{\ddagger}$ versus 29.1% for enzyme type) and $T_{opt}$ (33.5–60.7 °C) and $TS_{max}$ (18.2–40.3 °C) varying with both isolate and enzyme. The mechanistic reading of $\Delta C_p^{\ddagger}$ as a flexibility proxy rests on structural-biochemistry work: directed evolution of a designed enzyme generated increasingly negative activation heat capacities as activity improved (Bunzel et al. 2019), rigidifying a de novo enzyme induced a negative $\Delta C_p^{\ddagger}$ (Hindson et al. 2021), and high-precision kinetic data, molecular dynamics simulations, and crystallography on a model glycosidase support cooperative conformational restriction as the origin of $\Delta C_p^{\ddagger}$ (Walker et al. 2024; Arcus and Mulholland 2020). At the same time, Tang and Riley (2024) developed a chemical kinetics theory showing that substrate availability alone can shift the emergent temperature optimum of an enzyme-catalyzed reaction by up to about 20 K, warning that ignoring this can confound inferences of microbial thermal adaptation.

Problem Statement

For soil microbial extracellular enzymes characterized with macromolecular rate theory, determine (i) the mechanisms responsible for the observed differences in the temperature-sensitivity parameters — the heat capacity of activation ($\Delta C_p^{\ddagger}$), the temperature optimum ($T_{opt}$), and the temperature of maximum sensitivity ($TS_{max}$) — specifically, the relative contributions of enzyme flexibility, substrate type (the identity of the substrate acting on the enzyme-substrate complex, as distinct from substrate availability, which can shift the emergent optimum), genetic variation among taxa, and the isoenzyme composition of the assayed enzyme mixtures (the original measurements may average several isoenzymes rather than single purified enzymes), with a supported direction and relative contribution for each of these mechanisms where the evidence permits; (ii) how experimental conditions such as pH could shift the temperature-response curve and its MMRT parameters; and (iii) whether these traits acclimate or adapt as climates warm and what evolutionary constraints govern their change through time. Parts (i)-(iii) are each independently resolvable; how broadly the parameters vary across substantially wider phylogenetic and enzymatic diversity than the seven isolates and three enzyme types of the original study is outside the question posed here.

References

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