What does this tell you about the active site of the enzyme RuBisCO and what it likely binds in the substrates?

RuBisCO, the enzyme that fixes carbon in the Calvin Cycle, evolved at a time when there was less O₂ present on the earth.  Unfortunately, when the O₂ concentration is high, RuBisCo can bind to O₂ instead of CO₂ and make compounds that cannot be used in the Calvin Cycle.  This is called photo-respiration.  What does this tell you about the active site of the enzyme RuBisCO and what it likely binds in the substrates?