The 2019 coronavirus (SARS-CoV-2) surface protein (Spike) S1 Receptor Binding Domain undergoes conformational change upon heparin binding

Courtney Mycroft-West; Dunhao Su; Stefano Elli; Scott Guimond; Gavin Miller; Jeremy Turnbull; Edwin Yates; Marco Guerrini; David Fernig; Marcelo Lima; Mark Skidmore

The circular dichroism (CD) spectra of the SARS-CoV-2 S1 RBD in PBS was recorded using a J-1500 Jasco CD spectrometer, Spectral Manager II software and a 0.2 mm pathlength quartz cuvette (Hellma, USA) scanning at 100 nm.min -1 with a 1 nm resolution throughout the range of λ = 190 -260 nm. All spectra obtained were the mean of five independent scans post calibration with camphorsulfonic acid. SARS-CoV-2 S1 RBD was buffer-exchanged (prior to spectral analysis) using a 5 kDa Vivaspin centrifugal filter (Sartorius, Germany) at 12,000 g, thrice. Collected data was analysed with Spectral Manager II software prior to processing with GraphPad Prism 7, using a second order polynomial smoothing through 21 neighbours. Secondary structural prediction was calculated through the BeStSel analysis server 10 .To ensure the CD spectral change of SARS-CoV-2 S1 RBD in the presence of heparin was not altered owing to the addition of the heparin alone, which is known to possess CD spectra at high concentrations 11,12 a differential spectrum was analysed. The theoretical, summative CD spectra was confirmed to differ from the observed experimental CD spectra, thereby indicating that the change in the CD spectra compared to that of SARS-CoV-2 S1 RBD alone is a result of a conformational change upon binding to porcine mucosal heparin (Leo Pharma, UK).