Researchers clarify structure and function of new enzyme that reduces sulfite even faster

February 4, 2015, Albert Ludwigs University of Freiburg
The enzyme complex MccA contains ion and copper ions. Credit: AG Einsle

Sulfites are sulfurous substances that occur naturally. They are poisonous for many life forms even at small concentrations. Sulfites and sulfur dioxide are also added to wine and dried fruit as preservatives that inhibit the growth of unwanted microorganisms, increasing the shelf-life of these products. The biochemists Prof. Dr. Oliver Einsle and Dr. Bianca Hermann from the University of Freiburg have teamed up with researchers from the Technische Universität Darmstadt for a project in which they characterised a bacterial enzyme that reduces sulfite up to one hundred times faster than any other known enzyme.

The researchers were able to clarify the high-resolution crystal structure of the enzyme complex MccA and the molecular details of its reaction mechanism. Their findings explain why MccA is able to convert sulfite so quickly, meaning that this enzyme could be used more frequently in biotechnology in the future. Customised microorganisms capable of high-speed sulfite reduction could be employed for desulfurization under mild conditions, for example.

The results of the scientists' research have now been published in Nature.

Some bacteria use sulfite for their metabolism - for example, for respiration when there is no oxygen. Bacteria harvest energy by reducing sulfite to sulfide, during which they split off all oxygen atoms as water. One of the bacteria that use sulfur for respiration is Wolinella succinogenes. This organism can be found in the rumen of ruminants, where it produces MccA. MccA is a metalloprotein that contains 24 haem groups with iron ions. It also consists of three subunits, making it a trimer.

The scientists were able to decode the enzyme's structure, revealing a previously unknown active centre for sulfite reduction. This centre consists of one of the haem groups combined with a , which is bound to two residues of cysteine, a sulfurous amino acid. Through its position, the copper ion prevents sulfite ions from binding to the enzyme. Instead, binds to the active centre because it formally contains one less water molecule and thus requires less space, so to say. This initial step is what distinguishes the reduction mechanism of MccA from other enzymes that break down sulfite. MccA then reduces sulfur dioxide to sulfide through further dehydration. The scientists were able to detect sulfur dioxide and sulfur monoxide in the 's structural model, from which they concluded this new mechanism.

Explore further: A database of enzyme diversity

More information: Bianca Hermann, Melanie Kern, Luigi La Pietra, Jörg Simon & Oliver Einsle. 2015. "The octahaem MccA is a haem c-copper sulfite reductase." Nature. DOI: 10.1038/nature14109

Related Stories

A database of enzyme diversity

November 14, 2014

Scientists have a constructed a new database of the diversity in an enzyme that is used by microorganisms to metabolize sulfur.

Iron-sulfur enzymes as candidates for antibiotic development

October 9, 2012

The iron-sulfur protein IspH plays a central role in the terpene metabolism of several pathogens. The mechanism of the reaction provides an approach for developing new antibiotics, particularly against malaria and tuberculosis. ...

'Global positioning' for molecules

December 19, 2014

In everyday life, the global positioning system (GPS) can be employed to reliably determine the momentary location of one en route to the desired destination. Scientists from the Institute of Physical and Theoretical Chemistry ...

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.