In the Carbohydrate-Active Enzyme (CAZy) database, glycoside hydrolase family 5 (GH5) is a large family with more than 6,000 sequences. Among the 51 described GH5 subfamilies, sub-family GH5_26 contains members that display either endo-(1,4)-glucanase or (1,3;1,4)-glucanase activities. In this study, we focused on the GH5_26 enzyme from Saccharopha-gus degradans (SdGluc5_26A), a marine bacterium known for its capacity to degrade a wide diversity of complex polysaccha-rides. SdGluc5_26A displays lichenase activity toward (1,3; 1,4)-glucans with a side cellobiohydrolase activity toward (1, 4)-glucans. The three-dimensional structure of SdGluc5_26A adopts a stable trimeric quaternary structure also observable in solution. The N-terminal region of SdGluc5_26A protrudes into the active site of an adjacent monomer. To understand whether this occupation of the active site could influence its activity, we conducted a comprehensive enzymatic characterization of SdGluc5_26A and of a mutant truncated at the N terminus. Ligand complex structures and kinetic analyses reveal that the N terminus governs the substrate specificity of SdGluc5_26A. Its deletion opens the enzyme cleft at the 3 subsite and turns the enzyme into an endo-(1,4)-glucanase. This study demonstrates that experimental approaches can reveal structure-function relationships out of reach of current bioinformatic predictions.