Catalyzes a two-step oxygenase reaction involved in the synthesis of chlorophyll b. Acts specifically on the non-esterified chlorophyllide a and not on chlorophyll a. chlorophyllide a + 2 H(+) + 2 NADPH + 2 O2 = chlorophyllide b + 3 H2O + 2 NADP(+) Expressed in leaves and germinating seedlings, but not in sheaths and roots. Down-regulated by treatment with H(2)O(2). Consists of three domains A, B and C. The C-terminal C domain possesses catalytic function while the N-terminal A domain confers protein instability in response to chlorophyll b accumulation.