Evolution of a Catalytic Mechanism

Area of Science:

• Biochemistry
• Enzymology
• Molecular Evolution

Background:

• The origin of enzyme superfamilies via gene duplication and functional divergence is not well understood.
• The 'escape from adaptive conflict' hypothesis suggests ancestral enzymes were inefficient generalists, gaining specificity through gene duplication.
• Resurrected ancestral enzymes often show greater substrate promiscuity than modern counterparts, supporting this hypothesis.

Purpose of the Study:

• To provide evidence for the alternative 'innovation-amplification-divergence' hypothesis.
• To investigate the catalytic mechanisms and evolutionary history of plant esterases and acetone cyanohydrin lyases.
• To challenge the 'escape from adaptive conflict' model by examining ancestral enzyme properties.

Main Methods:

• Resurrection and characterization of ancestral plant esterases.
• Comparative analysis of catalytic mechanisms between ancestral and modern enzymes.
• Assessment of substrate specificity and promiscuity in ancestral and descendant enzymes.

Main Results:

• Catalytic mechanisms of plant esterases and acetone cyanohydrin lyases are incompatible, requiring different substrate binding orientations.
• Resurrected ancestral plant esterases exhibit high catalytic specificity, comparable to modern esterases.
• The ancestor of acetone cyanohydrin lyases possessed only weak promiscuous activity, with improvements in lyase function occurring at the expense of esterase activity.

Conclusions:

• The findings support the 'innovation-amplification-divergence' hypothesis, where a specific ancestor acquires a weak promiscuous activity that is then amplified.
• This contrasts with the 'escape from adaptive conflict' model, which posits a gradual loss of promiscuity from an inefficient generalist ancestor.
• Enzyme evolution may involve the amplification of a novel, weak activity at the cost of the original function, rather than the refinement of a generalist ancestor.