Endo-1,4-mannanase that catalyzes the random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans and heteromannans. It is a crucial enzyme for depolymerization of seed galactomannans and wood galactoglucomannans. Hydrolyzes structurally different mannan polysaccharides, such as galactomannans, glucomannans, and beta-1,4-mannans from different sources, yielding principally mannobiose (PubMed:21037302). Also has transglycosylation activity (PubMed:23558681). Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 4.0. Optimum temperature is 60 degrees Celsius. Monomer. Not glycosylated. Belongs to the glycosyl hydrolase 5 (cellulase A) family.