Are the y-axis values informative when performing biolayer interferometry?

Question

I am currently reading an this article by Alexandra C. Walls et. al.

I would like to ask a question about a graph that is being used in the article and I wanted to know if my analysis was correct.

I was wondering if for the Binding Response versus Time graphs of SARS-CoV-2 (panel A), and SARS-CoV-1 (panel B), can we infer that SARS-CoV-2 was more adept at binding to the human ACE2 because the y-axis has larger numbers than for SARS-CoV-1? If not, why not?

Answer 1

From the linked article:

We found that hACE2 bound to SARS-CoV-2 S^B and SARS-CoV S^B with respective equilibrium dissociation constants of 1.2 nM (Figure 2A) and 5.0 nM (Figure 2B).

An equilibrium dissociation constant (commonly called K_d) represents the propensity of a complex to dissociate into its constituent pieces. A smaller value represents a smaller propensity to break apart, i.e. a stronger interaction.

So, yes, considering only the data presented, domain B from the SARS-CoV-2 spike protein shows a slightly higher affinity for hACE2 than the equivalent domain from SARS-CoV. Note, however, that the authors are careful with their wording, and state in multiple places that the relative affinities of the homologous domains are comparable.

The SARS-CoV-2 S^B engages human ACE2 (hACE2) with comparable affinity to SARS-CoV S^B from viral isolates associated with the 2002–2003 epidemic (i.e., binding with high affinity to hACE2).

What's more telling are the binding constants (k_on and k_off) given in Table 1:

$$\begin{array}{c|c|c|} \text{} & \text{SARS-CoV-2 S}^{B} & \text{SARS-CoV S}^{B} \\ \hline \text{K}_{D} \text{ (nM)} & 1.2 ± 0.1 & 5.0 ± 0.1 \\ \hline \text{k}_{on} \text{ (M}^{-1} \text{s}^{-1}\text{)} & 1.4 × 10^5 (2.3 ± 1.4 × 10^5) & 1.4 × 10^5 (1.7 ± 0.7 × 10^5) \\ \hline \text{k}_{off} \text{(s}^{-1}\text{)} & 1.6 × 10^{−4} (1.7 ± 0.8 × 10^{−4}) & 7.1 × 10^{−4} (8.7 ± 5.1 × 10^{−4}) \\ \hline \end{array}$$ Values reported represent the global fit to the data shown in Figures 2A and 2B and the averages obtained from five (SARS-CoV-2) or four (SARS-CoV) replicates carried out with different protein preparations are shown in parentheses.

Note that K_d is the reciprocal of the association constant, K_a, which is equal to the ratio of the binding constants.

$K_{a} = \frac{k_{on}}{k_{off}}$, $K_{d} = \frac{k_{off}}{k_{on}}$

Since the values given for k_on derived from the global fit are the same for SARS-CoV-2 and SARS-CoV, the observed difference in K_d is due to the different values of k_off. So, at equilibrium, SARS-CoV-2 S^B and SARS-CoV S^B will associate with hACE2 at the same rate, though SARS-CoV S^B dissociates faster.