Biotransformations of carboxylated aromatic compounds by the acetogen Clostridium thermoaceticum: generation of growth-supportive CO2 equivalents under CO2-limited conditions

Abstract

Clostridium thermoaceticum ATCC 39073 converted vanillate to catechol. Although carboxylated aromatic compounds which did not contain methoxyl groups were not by themselves growth supportive, protocatechuate and p-hydroxybenzoate (nonmethoxylated aromatic compounds) were converted to catechol and phenol, respectively, during carbon monoxide-dependent growth. Syringate is not subject to decarboxylation by C. thermoaceticum (Z. Wu, S. L. Daniel, and H. L. Drake, J. Bacteriol. 170:5705-5708, 1988), and sustained growth at the expense of syringate-derived methoxyl groups was dependent on supplemental CO2. In contrast, vanillate was growth supportive in the absence of supplemental CO2, and 14CO2 was the major 14C-labeled product during [carboxyl-14C]vanillate-dependent growth. Furthermore, the decarboxylation of protocatechuate and p-hydroxybenzoate supported methanol- and 1,2,3-trimethoxybenzene-dependent growth (CO2 is required for growth at the expense of these substrates) when supplemental CO2 was depleted from the growth medium, and the decarboxylation of protocatechuate was concomitant with improved cell yields of methanol cultures. These findings demonstrate that (i) C. thermoaceticum is competent in the decarboxylation of certain aromatic compounds and (ii) under certain conditions, decarboxylation may be integrated to the flow of carbon and energy during acetogenesis.

Expression of an aromatic-dependent decarboxylase which provides growth-essential CO2 equivalents for the acetogenic (Wood) pathway of Clostridium thermoaceticum

“…Syringate is not subject to decarboxylation by C. thermoaceticum (16). Significantly, syringate-cultivated cells did not display any decarboxylase activity (Table 1); neither could 4-hydroxybenzoate-induced cells decarboxylate syringate (see below and Table 3).…”

“…In this regard, recent findings suggest that acetogens are metabolically integrated in nature to the anaerobic biotransformation of lignin-derived aromatic compounds (3,9,42). Studies to date indicate that acetogens contribute to this process by integrating the following aromatic substituents into the flow of both carbon and energy during acetogenesis: methoxyl groups (2, 6-8, 11, 21), acrylate groups (14,37), aldehydes (28), and carboxyl groups (16).…”

“…When larger quantities of cells were required, the above protocol was scaled up to 160-ml serum bottles. Stock solutions of aromatic compounds were prepared as previously described (16). The aromatic compounds studied were stable under the conditions utilized; transformations were not observed in the absence of cells.…”

“…Given the potential for the acetogen Clostridium thermoaceticum to derive growth-essential CO2 equivalents from carboxylated aromatic compounds, it has been proposed that some acetogens may initiate the anaerobic biotransformation of carboxylated aromatic compounds (16). To better understand the MATERIALS AND METHODS Cultivation of C. thermoaceticum.…”

“…RESULTS Expression of the aromatic-dependent decarboxylase. Previous studies established the potential of C. thermoaceticum to utilize the carboxyl groups of various aromatic compounds, including 4-hydroxybenzoate, when cultivated at the expense of CO (16). By using 4-hydroxybenzoate as the substrate, an assay (see Materials and Methods) was developed to assess the expression and activity of the aromaticdependent decarboxylase.…”