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Structure and function of highly active bacterial enzyme elucidated

A team of microbiologists from the TU Darmstadt and biochemists from the University of Freiburg, Germany, have jointly succeeded in determining the high-resolution crystal structure of a sulfite-reducing enzyme complex and elucidated details of its molecular reaction mechanism. This research potentially signifies a step forward in the field of biotechnology - tailor-made microorganisms could be utilised in the process of flue-gas desulferization at power plants.

Sulfites are naturally occurring substances that are toxic to many organisms, even at relatively low concentrations. This is due to their high degree of reactivity with the fundamental building blocks of organic cells, such as proteins, nucleic acids, and lipids. For this reason, sulfites have long been used to arrest the growth of undesirable microorganisms, for example, in order to increase the shelf-life of wine or dried fruits.

In addition, sulfites play an important role in the biogeochemical sulfur cycle as well as the field of atmospheric chemistry, and they are central metabolites in the microbial metabolism of sulfur compounds. For example, sulfite is a reaction intermediate of sulfate-reducing organisms. Apart from this, a number of bacteria exist that use sulfite for their energy metabolism and generate the necessary energy for growth from the reduction of sulfite to sulfide.

Efficient enzymatic sulfite reduction at an atomic level

This process is a type of so-called anaerobic respiration, in which the cell's central energy source, adenosine triphosphate, is generated by means of an electron transport chain, which in turn produces an electrochemical proton potential across the cell membrane. A typical model organism for sulfite respiration is the bacterium Wolinella succinogenes, which naturally occurs in the bovine rumen.

To date, various bacteria-derived sulfite-reducing enzymes have been identified; however, all have a relatively low conversion rate. In this aspect, the newly characterised enzyme purified from W. succinogenes is different. It is capable of reducing sulfite up to 100 times faster (up to 200 sulfite molecules per second). This enzyme is a metalloprotein that contains eight tightly-bound haem groups and is located between the cytoplasmic and the outer membrane of bacterial cells, where it forms a homotrimeric complex.

Microbiologists at the Technische Universität Darmstadt (research group led by Professor Jörg Simon) and biochemists at the University of Freiburg (research group led by Professor Oliver Einsle) have now been able to determine the high-resolution crystal structure of this enzyme complex and to elucidate details of its molecular reaction mechanism. The structure of the trimeric enzyme, which contains 24 haem groups, reveals an impressive and hitherto unknown active site of sulfitereduction.

Future challenges

This catalytic centre is composed of a haem group and a juxtaposed copper ion, which itself is bound via two cysteine residues. The position of the otherwise redox-inactive copper ion prevents the sulfite anion from binding to the enzyme. It does not, however, hamper binding of its dehydration product, sulfur dioxide, as could be shown in the structural model. In addition, the researchers were able to demonstrate the existence of the primary reduction product, sulfur monoxide, and to postulate a model of the complete reaction mechanism for the reduction pathway from sulfitetosulfide.

The data that were produced reveal the atomic image of a novel haem-copper enzyme, which is able to explain the high speed of the sulfite conversion and which brings the possible use of this enzyme in the biotechnology sector one step closer. For example, it may be conceivable to use microorganisms that are capable of the rapid reduction of sulfite or sulfur dioxide in the desulfurization of flue-gases under mild conditions. In future, this idea as well as its technical implementation will be pursued within the Research Focus for Synthetic Biologybased at the Department of Biology at the Technische Universität Darmstadt.

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Source: TU Darmstadt