Copper Binding before Polypeptide Folding Speeds Up Formation of Active (Holo) Pseudomonas aeruginosa Azurin

Cofactors often stabilize the native state of the proteins; however, their effects on folding dynamics remain poorly understood. To uncover the role of one cofactor, we have examined the folding kinetics of Pseudomonas aeruginosa azurin, a small blue-copper protein with a copper cofactor uniquely coordinated to five protein residues. Copper removal produces apo-azurin which adopts a folded structure identical to that of the holo-form. The folding and unfolding kinetics for apo-azurin follow two-state behavior. The extrapolated folding time in water, τ ∼ 7 ms, is in good agreement with the topology-based prediction. Copper uptake by folded apo-azurin, to govern active (holo) protein, is slow (τ ∼ 14 min, 50:1 copper-to-protein ratio). In contrast, the formation of active (holo) azurin is much faster when copper is allowed to interact with the unfolded polypeptide. Refolding in the presence of 10:1, 50:1, and 100:1 copper:protein ratios yields identical time-trajectories: active azurin forms in two kinetic phases with folding times, extrapolated to water, of τ = 10 ± 2 ms (major phase) and τ = 190 ± 30 ms (minor phase), respectively. Correlating copper-binding studies, with a small peptide derived from the metal-binding region of azurin, support that initial cofactor binding is fast (τ ∼ 3.7 ms) and thus not rate-limiting. Taken together, introducing copper prior to protein folding does not speed up the polypeptide-folding rate; nevertheless, it results in much faster (> 4000-fold) formation of active (i.e., holo) azurin. Living systems depend on efficient formation of functional biomolecules; attachment of cofactors prior to polypeptide folding appears to be one method to achieve this.