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Collagen is the most abundant mammalian protein and is a critical component of the extracellular matrix. But a new study reveals that collagen is unstable at body temperature.For decades, it has been thought that the helices of collagen have a melting temperature just a few degrees above body temperature. Now, however, Sergey Leikin (National Institutes of Health, Bethesda, MD) and colleagues have used modern calorimetry techniques to show that the equilibrium melting temperature (Tm) of type I collagen is lower than previously thought— several degrees lower than body temperature, in fact.2 n+ H- C, x1 J" G: k1 W
# k5 l: a! T4 y5 o+ e"Our results are consistent with all of the previous data," Dr. Leikin points out, "except we have expanded the range of measurements" by increasing the equilibration time by several orders of magnitude. By measuring collagen denaturation at a much slower heating rate, the group was able to confirm a previous report that the apparent Tm has a logarithmic, not linear, dependence on the heating rate. Since prior calculations of the equilibrium Tm of collagen were based on extrapolations of a linear dependence, these calculations were inaccurate.+ ~+ H& l# Q8 d' @! U
1 P0 Z5 p* v/ m9 H9 ]The new data indicate that, at body temperature, the preferred conformation of collagen is a random coil rather than a triple helix, as previously thought. Folding inside cells may be mediated through chaperones. This intrinsic instability means that once collagen is secreted from cells it begins to unfold. Partial unfolding may be essential for the protein to undergo fiber formation, which then stabilizes the helices of the protein and prevents further unfolding. However, the low melting temperature allows collagen molecules to melt and refold locally, providing elasticity and strength to the fibers. Additionally, defective or unused collagen molecules may denature relatively rapidly, allowing for quick degradation by proteolytic enzymes. ' f" q2 z7 x# ` t$ k/ B* P5 T `- F 0 a8 ?8 R6 ^- T/ T5 Z7 a% b! }6 LReference: & [5 q ^8 V, f4 a5 H$ \" A& Y/ p 1 \. n* v4 C; kLeikina, E., et al. 2002. Proc. Natl. Acad. Sci. USA. doi 10.1073/pnas.032307099 ) }; p4 T0 w+ _/ T . ?$ S. _( A7 |( K* \ \labrasn@rockefeller.edu(At longer equilibration times, collagen )作者: marysyq 时间: 2015-5-24 19:20