This study aimed to evaluate the effect of the 5E inquiry-based instruction on the improvement of Grade 11 students' conceptual understanding and attitude towards learning enzymes at Fasilo Secondary School, Bahir Dar, Ethiopia. A Design-Based Research (DBR) framework was employed, involving iterative cycles of design, implementation, analysis, and redesign. The DBR cycles were used to iteratively design and refine the intervention, while a quasi-experimental embedded mixed-methods design provided rigorous evidence of its effectiveness. Two intact classes (N=38) were assigned to experimental and control groups. Data were collected using the Enzymes Concept Test (ECT), Biology Attitude Scale (BAS), classroom observations, semi-structured interviews, and student artifacts. Quantitative data were analyzed using descriptive statistics, ANCOVA, Mann-Whitney U test, and Wilcoxon signed-rank test. Qualitative data were analyzed thematically. The results revealed that both groups had identical pre-test conceptual understanding (M=4.158), but the experimental group's post-test mean (M=6.947) was significantly higher than the control group's (M=5.211), F(1,35)=879.70, p<.001, η²=.962. Regarding attitude, the experimental group showed significantly higher post-test ranks (Mean Rank=28.34) than the control group (Mean Rank=10.66), U=12.50, Z=-4.966, p<.001. Within the experimental group, 18 out of 19 students showed improved attitudes (Z=-3.742, p<.001). The BAS demonstrated excellent reliability (α=.970). Student perceptions were overwhelmingly positive, with 84.2% liking the 5E method and 100% agreeing hands-on activities helped understanding. The study concludes that the locally adapted 5E instructional model significantly improves both conceptual understanding and attitude towards learning enzymes. Recommendations include improving the integration of the 5E model into the biology curriculum, providing teacher professional development, and replicating the DBR approach for other challenging science topics.