Glycogen is a central energy storage molecule in bacteria and the metabolic pathways associated with its biosynthesis and degradation are crucial for maintaining cellular energy homeostasis. In mycobacteria, the GlgE pathway involves the combined action of trehalose synthase (TreS), maltokinase (Mak) and maltosyltransferase (GlgE). The N-terminal lobe can be divided into two subdomains: a cap N-terminal subdomain comprising the first 88 amino acid residues. This entry is for the cap N-terminal domain found in mycobacterial maltokinase (Mak), (E). The N-terminal cap subdomain and the C-terminal lobe are predominantly acidic, the intermediate subdomain is enriched in positively charged residues. A structural search with only the first 88 amino acid residues of Mak, corresponding to the N-terminal cap subdomain of maltokinases, unveiled a resemblance with proteins displaying the cystatin fold and a remote similarity with the N-terminal domain of the serine/threonine protein kinase GCN2. Conservation of the cap subdomain in maltokinases (including the bifunctional TreS-Mak enzymes), in particular of the residues in the proximity of the P-loop, together with the potential flexibility of this region, are compatible with regulatory functions for this subdomain. Hence it is hypothesised that the N-terminal cap subdomain plays a central role in modulation of Mak enzymatic activity [].
