Hidden Markov Modeling with Detailed Balance and Its Application to Single Protein Folding

Hidden Markov modeling (HMM) has revolutionized kinetic studies of macromolecules. However, results from HMM often violate detailed balance when applied to the transitions under thermodynamic equilibrium, and the consequence of such violation has not been well understood. Here, to our knowledge, we developed a new HMM method that satisfies detailed balance (HMM-DB) and optimizes model parameters by gradient search. We used free energy of stable and transition states as independent fitting parameters and considered both normal and skew normal distributions of the measurement noise. We validated our method by analyzing simulated extension trajectories that mimicked experimental data of single protein folding from optical tweezers. We then applied HMM-DB to elucidate kinetics of regulated SNARE zippering containing degenerate states. For both simulated and measured trajectories, we found that HMM-DB significantly reduced overfitting of short trajectories compared to the standard HMM based on an expectation-maximization algorithm, leading to more accurate and reliable model fitting by HMM-DB. We revealed how HMM-DB could be conveniently used to derive a simplified energy landscape of protein folding. Finally, we extended HMM-DB to correct the baseline drift in single-molecule trajectories. Together, we demonstrated an efficient, versatile, and reliable method of HMM for kinetics studies of macromolecules under thermodynamic equilibrium.