Two states or not two states: Single-molecule folding studies of protein L.

Experimental tools of increasing sophistication have been employed in recent years to study protein folding and misfolding. Folding is considered a complex process, and one way to address it is by studying small proteins, which seemingly possess a simple energy landscape with essentially only two stable states, either folded or unfolded. The B1-IgG binding domain of protein L (PL) is considered a model two-state folder, based on measurements using a wide range of experimental techniques. We applied single-molecule fluorescence resonance energy transfer (FRET) spectroscopy in conjunction with a hidden Markov model analysis to fully characterize the energy landscape of PL and to extract the kinetic properties of individual molecules of the protein. Surprisingly, our studies revealed the existence of a third state, hidden under the two-state behavior of PL due to its small population, ∼7%. We propose that this minority intermediate involves partial unfolding of the two C-terminal ß strands of PL. Our work demonstrates that single-molecule FRET spectroscopy can be a powerful tool for a comprehensive description of the folding dynamics of proteins, capable of detecting and characterizing relatively rare metastable states that are difficult to observe in ensemble studies.

Direct observation of ultrafast large-scale dynamics of an enzyme under turnover conditions.

The functional cycle of many proteins involves large-scale motions of domains and subunits. The relation between conformational dynamics and the chemical steps of enzymes remains under debate. Here we show that in the presence of substrates, domain motions of an enzyme can take place on the microsecond time scale, yet exert influence on the much-slower chemical step. We study the domain closure reaction of the enzyme adenylate kinase from Escherichia coli while in action (i.e., under turnover conditions), using single-molecule FRET spectroscopy. We find that substrate binding increases dramatically domain closing and opening times, making them as short as ∼15 and ∼45 µs, respectively. These large-scale conformational dynamics are likely the fastest measured to date, and are ∼100-200 times faster than the enzymatic turnover rate. Some active-site mutants are shown to fully or partially prevent the substrate-induced increase in domain closure times, while at the same time they also reduce enzymatic activity, establishing a clear connection between the two phenomena, despite their disparate time scales. Based on these surprising observations, we propose a paradigm for the mode of action of enzymes, in which numerous cycles of conformational rearrangement are required to find a mutual orientation of substrates that is optimal for the chemical reaction.

In vitro suppression of two different stop codons.

Proteins play a crucial role in all living organisms, with the 20 natural amino acids as their building blocks. Unnatural amino acids are synthetic derivatives of these natural building blocks. These amino acids have unique chemical or physical properties as a result of their specific side chain residues. Their incorporation into proteins through ribosomal translation in response to one of the stop codons has opened a new way to manipulate and study proteins by enabling new functionalities, thus expending the genetic code. Different unnatural amino acids have different functionalities, hence, the ability to incorporate two different unnatural amino acids, in response to two different stop codons into one protein is a useful tool in protein manipulation. This ability has been achieved previously only in in vivo translational systems, however, with limited functionality. Herein, we report the incorporation of two different unnatural amino acids in response to two different stop codons into one protein, utilizing a cell-free protein synthesis system. Biotechnol. Bioeng. 2017;114: 1065-1073. © 2016 Wiley Periodicals, Inc.