use 6 expression and characterization of recombinant human insulin b 1 2 k b 4 28 e e a 1 20 g precursor recombinant human insulin b 1 2 k b 4 28 e e a 1 20 g precursor was expressed and identified as in example 1 the results showed that clone no 093 15 expressed the protein of interest and was selected for following fermentation clone no 093 15 was selected for fermentation the fermentation method was as described in example 1 the fermentation product was isolated and purificated as described in example 1 the resulting expression product was as follows the molecular weight of the obtained product detected by lc ms was 6060 which is consistent with the theoretically predicted molecular weight of 6060 methods of enzymatic digestion and purification were as described in example 1 after optimization of the enzymatic reaction system digested products prepared by reverse phase were as follows the molecular weight of the obtained product detected by lc ms was 5793 which is consistent with the theoretically predicted molecular weight of 5793 6 the structure of disulfide bonds in cleaved products was analyzed as described in example 1 results obtained from analysis of disulfide bond structure in human insulin b 1 2 k b 4 28 e e.