Mechanics and structure of cross-bridges during contractions initiated by photolysis of caged Ca2+

Abraham Patchornik: The Contemporary Relevance of His Work for Chemistry and Biology

Abraham Patchornik was born in 1926 in Ness Ziona, a town in Palestine founded by his great-grandfather Reuben Lehrer in 1883. He started to study chemistry as an undergraduate at the Hebrew University. However, this was interrupted by the war, and he completed his studies in various locations in West Jerusalem. From 1952 to 1956 Patchornik completed his PhD at the (new) Weizmann Institute of Science with Ephraim Katchalski. After a postdoc at the NIH, he returned to the Weizmann in 1958, when he joined the Department of Biophysics. In 1972-1979, he became chairman of the new Department of Organic Chemistry at the Weizmann, and his own research was geared toward applying creative chemistry to solve biological problems. Patchornik passed away in his hometown of Ness Ziona in 2014. Patchornik was a conceptual leader in peptide and polymer chemistry. Given the importance of selective functional group protection for the construction of oligomeric molecules, he became interested in using "nonstandard", orthogonal chemistry for this purpose, i.e. photosensitive protecting groups (PPGs) in place of thermal reactions. It was R.B. Woodward who suggested this strategy to Patchornik in 1965, while Patchornik was on sabbatical leave at Harvard. However, it was not until Patchornik returned to the Weizmann that this idea of a versatile PPG to enable multistep synthesis was realized. Here, we provide an account of the early photosensitive protecting groups that Patchornik and co-workers developed, and the immense impact they have had on various fields. In particular, we survey the use of PPGs in live cell physiology (i.e., caged compounds), and the development of gene chips via light-directed solid-phase synthesis. Further, we highlight recent work applying new PPGs for "photochemical delivery" of drugs, otherwise termed photopharmacology. Finally, we discuss the relationship between caged compounds and how contemporary neuroscience uses genetically encoded chromophores to control cell function.

Structure and periodicities of cross-bridges in relaxation, in rigor, and during contractions initiated by photolysis of caged Ca2+

Ultra-rapid freezing and electron microscopy were used to directly observe structural details of frog muscle fibers in rigor, in relaxation, and during force development initiated by laser photolysis of DM-nitrophen (a caged Ca2+). Longitudinal sections from relaxed fibers show helical tracks of the myosin heads on the surface of the thick filaments. Fibers frozen at approximately 13, approximately 34, and approximately 220 ms after activation from the relaxed state by photorelease of Ca2+ all show surprisingly similar cross-bridge dispositions. In sections along the 1,1 lattice plane of activated fibers, individual cross-bridge densities have a wide range of shapes and angles, perpendicular to the fiber axis or pointing toward or away from the Z line. This highly variable distribution is established very early during development of contraction. Cross-bridge density across the interfilament space is more uniform than in rigor, wherein the cross-bridges are more dense near the thin filaments. Optical diffraction (OD) patterns and computed power density spectra of the electron micrographs were used to analyze periodicities of structures within the overlap regions of the sarcomeres. Most aspects of these patterns are consistent with time resolved x-ray diffraction data from the corresponding states of intact muscle, but some features are different, presumably reflecting different origins of contrast between the two methods and possible alterations in the structure of the electron microscopy samples during processing. In relaxed fibers, OD patterns show strong meridional spots and layer lines up to the sixth order of the 43-nm myosin repeat, indicating preservation and resolution of periodic structures smaller than 10 nm. In rigor, layer lines at 18, 24, and 36 nm indicate cross-bridge attachment along the thin filament helix. After activation by photorelease of Ca2+, the 14.3-nm meridional spot is present, but the second-order meridional spot (22 nm) disappears. The myosin 43-nm layer line becomes less intense, and higher orders of 43-nm layer lines disappear. A 36-nm layer line is apparent by 13 ms and becomes progressively stronger while moving laterally away from the meridian of the pattern at later times, indicating cross-bridges labeling the actin helix at decreasing radius.

Structural changes in muscle crossbridges accompanying force generation

We have investigated the structure of the crossbridges in muscles rapidly frozen while relaxed, in rigor, and at various times after activation from rigor by flash photolysis of caged ATP. We used Fourier analysis of images of cross sections to obtain an average view of the muscle structure, and correspondence analysis to extract information about individual crossbridge shapes. The crossbridge structure changes dramatically between relaxed, rigor, and with time after ATP release. In relaxed muscle, most crossbridges are detached. In rigor, all are attached and have a characteristic asymmetric shape that shows strong left-handed curvature when viewed from the M-line towards the Z-line. Immediately after ATP release, before significant force has developed (20 ms) the homogeneous rigor population is replaced by a much more diverse collection of crossbridge shapes. Over the next few hundred milliseconds, the proportion of attached crossbridges changes little, but the distribution of the crossbridges among different structural classes continues to evolve. Some forms of attached crossbridge (presumably weakly attached) increase at early times when tension is low. The proportion of several other attached non-rigor crossbridge shapes increases in parallel with the development of active tension. The results lend strong support to models of muscle contraction that have attributed force generation to structural changes in attached crossbridges.