Structure and mechanism of action of two bacterial enzymes: MltE from Escherichia coli and AspB from Bacillus sp YM55-1

PhD ceremony: Mr. G. Fibriansah, 12.45 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Structure and mechanism of action of two bacterial enzymes: MltE from Escherichia coli and AspB from Bacillus sp YM55-1

Promotor(s): prof. B.W. Dijkstra

Faculty: Mathematics and Natural Sciences

In this thesis, crystal structures and reaction mechanisms of two different bacterial enzymes are described. The first enzyme is the lytic transglycosylase MltE from Escherichia coli, which cleaves the b-1,4-glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine residues in the bacterial cell wall material peptidoglycan. The enzyme is thought to function in bacterial cell wall turn-over, remodeling and maintenance, which makes it a potential target for antibacterials. MltE is distinct because it is one of the few endo-acting lytic transglycosylases of E. coli. The crystal structures of MltE in a substrate-free state, in a binary complex with chitopentaose, and in a ternary complex with the glycopeptide inhibitor bulgecin A and a murodipeptide allowed a detailed analysis of the saccharide-binding interactions. In combination with site-directed mutagenesis studies the structures explain why MltE is an endo-acting enzyme and how it catalyzes the reaction. The second enzyme is the aspartase AspB from Bacillus sp. YM55-1, which catalyzes the reversible deamination of L-aspartate into fumarate and ammonia. Aspartases are used as biocatalysts for the industrial production of enantiopure L-aspartate, an important starting compound for the synthesis of food additives and artificial sweeteners. However, their precise catalytic mechanism has remained elusive because of lack of information on the binding mode of substrate, product or substrate analogs. Crystal structures of AspB in an unliganded state and with bound L-aspartate have now revealed the residues responsible for catalysis. Accompanying site directed mutagenesis and enzyme kinetics experiments allowed to fully explain the mechanism of action of this enzyme.