Step 1: recall where metal complexes hide in the body.
Living systems quietly use coordination compounds for some of their most important jobs, where a central metal ion is wrapped by an organic framework. We just need to name two well-known ones.
Step 2: give the examples.
The first is chlorophyll, the green pigment of leaves, which is a complex of magnesium and captures sunlight to run photosynthesis. The second is haemoglobin, the red protein of blood, which is a complex of iron and ferries oxygen around the body. Vitamin $\mathrm{B_{12}}$, a cobalt complex, is a further example.
Two biologically important coordination compounds are chlorophyll (magnesium) and haemoglobin (iron).