Structural insights into the loss of catalytic competence in pectate lyase activity at low pH

Pectate lyase, a family 1 polysaccharide lyase, catalyses cleavage of the α-1,4 linkage of the polysaccharide homogalacturonan via an anti β-elimination reaction. In the Michaelis complex two calcium ions bind between the C6 carboxylate of the d-galacturonate residue and enzyme aspartates at the active centre (+1 subsite), they withdraw electrons acidifying the C5 proton facilitating its abstraction by the catalytic arginine. Here we show that activity is lost at low pH because protonation of aspartates results in the loss of the two catalytic calcium-ions causing a profound failure to correctly organise the Michaelis complex.