PMID: 25774984

Authors:

Mobbs JI, Koay A, Di Paolo A, Bieri M, Petrie EJ, Gorman MA, Doughty L, Parker MW, Stapleton D, Griffin MD, Gooley PR

Title:

Determinants of oligosaccharide specificity of the carbohydrate binding modules of AMP-activated protein kinase.

Journal:

Biochem J. 2015 Mar 16.

Abstract:

AMPK-activated protein kinase (AMPK) is a alphabetagamma heterotrimer that is important in regulating energy metabolism in all eukaryotes. The beta-subunit exists in two isoforms (beta1 and beta2) and contains a carbohydrate binding module (CBM) that interacts with glycogen. The two CBM isoforms (beta1- and beta2-CBM) are near identical in sequence and structure, yet show differences in carbohydrate binding affinity. beta2-CBM binds linear carbohydrates with four-fold greater affinity than beta1-CBM and binds single alpha1,6-branched carbohydrates up to 30-fold tighter. To understand these affinity differences, especially for branched carbohydrates, we determined the NMR solution structure of beta2-CBM in complex with the single alpha1,6 branched carbohydrate glucosyl-beta-cyclodextrin (gBCD) which supported the dynamic nature of the binding site, but resonance broadening prevented defining where the alpha1,6 branch bound. We therefore solved the X-ray crystal structures of beta1- and beta2-CBM, in complex with gBCD, to 1.7 A and 2.0 A respectively. The additional threonine (Thr-101) of beta2-CBM expands the size of the surrounding loop, creating a pocket that accommodates the alpha1,6 branch. Hydrogen bonds are formed between the alpha1,6 branch and the backbone of Trp-99 and Lys-102 side chain of beta2-CBM. In contrast the alpha1,6 branch could not be observed in the beta1-CBM structure, suggesting that it does not form a specific interaction. The orientation of gBCD bound to beta1- and beta2-CBM is supported by thermodynamic and kinetic data obtained through isothermal titration calorimetry and NMR. These results suggest that AMPK containing the muscle specific beta2-isoform may therefore have greater affinity to partially degraded glycogen.