Enzyme Classification: Lyase
The enzyme that catalyzes the cleavage of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate is a lyase (specifically, fructose-1,6-bisphosphate aldolase).
Rationale for Classification
The reaction shown depicts the reversible cleavage of fructose 1,6-bisphosphate (a 6-carbon sugar) into two 3-carbon products: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP). This is a carbon-carbon bond cleavage reaction that does not involve:
Lyase Mechanism
Lyases catalyze the cleavage of C-C, C-O, C-N, and other bonds by elimination, leaving double bonds or rings, or conversely adding groups to double bonds 1, 2. Fructose-1,6-bisphosphate aldolase specifically performs an aldol cleavage reaction, which is the hallmark of lyase activity 3.
Two Classes of Aldolase
While both perform the same lyase reaction, there are two mechanistically distinct classes:
- Class I aldolases: Use Schiff base formation with an active-site lysine residue to stabilize the reaction intermediate through covalent catalysis 1, 4
- Class II aldolases: Utilize a divalent metal ion (typically zinc) at the active site to polarize the carbonyl group and stabilize the carbanion intermediate 2, 5
Both classes catalyze the same reversible aldol cleavage/condensation reaction, making them lyases regardless of their mechanistic differences 5.
Key Distinguishing Features
The reaction involves:
- Carbon-carbon bond cleavage between C3 and C4 of fructose 1,6-bisphosphate 4
- No net oxidation or reduction of the substrate 1
- No water molecule participation in the bond-breaking step 2
- Stereospecific aldol cleavage producing specific stereoisomers of the products 4
Answer: B) Lyase