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. Active against locust bean gum and ivory nut mannan, releasing mainly tri- and disaccharides (PubMed:7793911, Ref.3, Ref.4, PubMed:8529653). Also has transglycosylation activity. Transglycosylation of two mannotrioses into a mannohexaose is the major transglycosylation route (PubMed:8529653, Ref.7, PubMed:24950755). Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 5.0. Stable from pH 2.5 to 7.0. Optimum temperature is 75 degrees Celsius. Monomer. The enzyme consists of two functional domains, a catalytic core joined to a carbohydrate-binding domain (CBM) by a serine-, threonine-, and proline-rich, highly glycosylated linker sequence (Probable). The CBM binds to cellulose but not to mannan, and increases the mannan-hydrolysis of complex substrates (PubMed:12523968). Belongs to the glycosyl hydrolase 5 (cellulase A) family.