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 gum guar (PubMed:16844780). Also has efficient transglycosylation activity. Produces mainly mannopentaose and mannoheptaose out of mannotetraose. May even prefer sugars as acceptors instead of water (PubMed:24950755). Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Optimum pH is 6. Optimum temperature is 60 degrees Celsius. Monomer. Belongs to the glycosyl hydrolase 5 (cellulase A) family.