Hen egg white lysozyme is a common example in textbooks discussing enzyme mechanism. But now scientists from the University of Bristol, UK, have used molecular dynamics simulations to show that the traditional mechanism is wrong. 'The textbooks need to be rewritten,' says researcher Adrian Mulholland, who led the team behind the work.

Lysozymes break down polysaccharides in bacterial cell walls, and so play a role in defence against pathogens. The textbook mechanism of hen egg white lysozyme, proposed in the 1960s, proceeds through an intermediate in which a sugar ring on the substrate interacts ionically with the enzyme. In the revised mechanism the bond is covalent. 

'Knowing how reaction intermediates form is central to understanding why enzymes are such efficient catalysts,' says Mulholland. 'This sort of detailed knowledge is also important in designing enzyme inhibitors as drugs,' he adds.

The hen egg white lysozyme mechanism proceeds through a covalently bound intermediate

A covalent intermediate has been suggested previously, clarifies Mulholland. But the experimental work that led to this proposal relied on modified enzymes and substrates because the wild type enzyme is too efficient for any intermediate to be detected. 'Some people have suggested that the experiments were not relevant to the real target,' Mulholland says.

Mulholland's computational model is based on the wild type enzyme and substrate. Moreover it includes the entire protein as well as its water environment, in contrast to previous smaller models. The evidence from the modelling and the experiments together is enough to confirm the revised mechanism, says Mulholland.

Stephen Withers was involved in the experimental research on hen egg white lysozyme, and is pleased that Mulholland's calculations have confirmed the revised mechanism. The scientist from the University of British Columbia in Vancouver, Canada, welcomes the development of computational studies to supplement experiments. 'It is simply not possible to experimentally probe all proteins,' he says. 'We shall rely on computational approaches increasingly to guide our choice of systems to study experimentally.'

Danièle Gibney