Table 2.
Kinetic and equilibrium constants for binding to POPC bilayers at room temperature. The data for the original peptides is from Almeida and Pokorny (6) and references therein. The estimated relative error in the rate and equilibrium constants is about 30%, except for δ-lysin, where it is larger (see footnote a).
| Peptide | KD (μM) | kon M−1s−1 | koff s−1 |
|---|---|---|---|
| δ-Lysina | 30 | 2 × 103 | 0.06 |
| DL-1 | 400 | 2.5 × 104 | 10 |
| DL-2a | 200 | 5.5 × 102 | 0.11 |
| DL-2bb | 3400 | 2.5 × 103 | 8.6 |
| Cecropin A | 1000 | 2.8 × 105 | 300 |
| CE-1 | 400 | 1.9 × 104 | 7.5 |
| CE-2 | 4700 | 3.0 × 104 | 140 |
| Magainin-2c | 2000 | 2.8 × 105 | 550 |
| MG-1 | 200 | 6.1 × 104 | 13 |
| MG-2 | 1100 | 2.1 × 104 | 24 |
Measuring binding for δ-lysin has been especially difficult, for reasons we do not fully understand. Data was collected previously, for POPC and other unsaturated phosphatidylcholines to which binding is similar (6, 59). On the basis of all those data, the best previous estimate was KD = 60 μM (6). We have now re-examined the earlier data, and supplemented it with new data (not shown). The numbers listed here are our improved estimates at this point. However, kon is strikingly small. From our previous estimates (6, 15), the monomer should be the dominant species in solution at a concentration of 0.5 μM, which was used in the binding kinetics. Nevertheless, the small kon suggests that either the monomer conformation in solution is such that binding is impaired, or that some peptide oligomerization, which is known to occur in solution above 1 μM (14, 54–57), persists at 0.5 μM. If the oligomerization equilibrium contributes to the measured on-rates, the true KD would be even smaller.
Data from only one experiment.
The binding data was obtained for the variant F12W of magainin 2, which, like the F16W variant, behaves very similarly to the original magainin (71). The value of KD = 5 mM (32) was revised. The new estimate of KD = 2 mM was obtained by direct extrapolation of the KD in POPC/POPG mixtures, as a function of POPG content in the membrane, to pure POPC. Previously, we had extrapolated the kon and koff, and obtained KD from their ratio (32), but kon had been overestimated. The new estimates of the rate constants for magainin 2 (F12W) in POPC are listed here.