Complexation of chitosan and pullulanase was affected by chitosan molecular weight.

Pullulanase was immobilized onto Fe₃O₄–κ-carrageenan nanoparticles by complexation.

Interrelationship exists between chitosan Mw and binding characteristic.

The high binding affinity induces more alterations of protein secondary structure.

Changes in secondary structures impact enzymatic properties of immobilized enzyme.