Lock and Key Model
The model proposing that the substrate fits a rigid, precisely shaped active site as a key fits a lock.
Unit
Dimensionless
In depth
Proposed by Emil Fischer in 1894, the model explains enzyme specificity simply and intuitively. Its weakness is the assumption of rigidity, which cannot account for the conformational changes now known to accompany substrate binding.
Examples in the real world
Fischer's explanation of why an enzyme acting on one sugar ignores its stereoisomer.