A functional fusion protein, which consists of an antibody and an enzyme that can be used in enzyme immunoassays, has been constructed. However, a quantitative comparison of the characteristics of fusion proteins and chemical conjugates of the parents, which are functionally produced in a uniform microbial system, has not been adequately achieved. In this study, a fusion protein between the ZZ protein and Escherichia coli alkaline phosphatase (AP) and the parental ZZ protein and AP for chemical conjugate was functionally produced in the same bacterial system. A detailed examination of the ZZ-AP fusion protein and the effect of the ZZ-AP chemical conjugate on IgG affinity and enzymatic activity were performed. Compared with the parents, the equilibrium dissociation constant of ZZ-AP conjugate decreased by 32 % and catalytic activity decreased by 24 %, whereas the ZZ-AP fusion retained full parental activities and exhibited an approximately tenfold higher sensitivity than that of ZZ-AP conjugate in enzyme-linked immunosorbent assay. Thus, ZZ-AP fusion is a promising immunoreagent for IgG detection and a potential biolinker between antibodies and reporter enzymes (i.e., IgG-ZZ-AP fusion complex) in immunoassays.