Endo-1,4-mannanase, a crucial enzyme for depolymerization of seed galactomannans and wood galactoglucomannans. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 4.0 to 5.0. Optimum temperature is 65 degrees Celsius. Stable up to 55 degrees Celsius. Has a modular structure: a carbohydrate-binding module (CBM) at the N-terminus, a linker rich in serines, and a C-terminal endo-1,4-mannanase catalytic module. The genes for catalytic modules and CBMs seem to have evolved separately and have been linked by gene fusion. Belongs to the glycosyl hydrolase 5 (cellulase A) family. Extended N-terminus.