Figure 4.

N439K creates a new RBD:hACE2 salt bridge and enhances RBD:hACE2 affinity

(A–C) X-ray structures of the SARS-CoV (A), SARS-CoV-2 WT (B), and SARS-CoV-2 N439K (C) RBD in complex with hACE2 (based on 2AJF, 6M0J, and current work, respectively). Select interface residues are shown as sticks. hACE2 is shown in orange and RBD in gray. The inset in (C) shows the 2Fo-Fc electron density contoured at 1σ for the K439-E329 salt bridge.

(D) Binding affinity of RBD and Spike variants for hACE2 measured by surface plasmon resonance. Monomeric hACE2 is injected successively at 11, 33, 100, and 300 nM onto surface-captured spike extracellular domain (ECD) or RBD; alternately, RBD is injected successively at 3.1, 12.5, and 50 nM onto surface-captured hACE2. All spike ECD contain the D614G mutation. Bar graph: affinity measurements (averages of 3–4 replicates) expressed as a fold change relative to WT binding within each experiment format, where >1 indicates improved binding (smaller K_D) relative to WT. WT K_D values measured as: 95 ± 1.6 nM (Spike surface), 63 ± 1.0 nM (RBD surface), 19 ± 3.3 nM (hACE2 surface); errors are SEM.

See also Table S3.