Contribution of carboxyl groups to heavy metal binding sites in fungal wall

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Abstract
Carboxyl groups contained in the cell walls of four different fungal species (Rhizopus arrhizus, Mucor miehei. Penicillium chrysogenum and Trichoderma reesei) were esterified with acidic methanol. The extent of esterification was monitored by measuring (gas chromatography) the methanol released after hydrolysis. The effect of biomass modification on metal binding was determined by zinc binding experiments at pH 5.5. A good correlation was observed between carboxyl blocking and reduction of zinc binding. Controls were also performed to examine the possible influence of methanol or acidic treatment alone. A good specificity was observed and the results demonstrate that carboxyl groups are responsible for at least 30% of R. arrhizus and M. miehei zinc binding properties. This contribution is higher with P. chrysogenum and T. reesei, with 55% and 70% of zinc binding, respectively. In addition, a non specific effect of methanol was observed with R. arrhizus, that strongly suggests that lipids contribute to a large extent (40%), to zinc binding by this fungal biomass.