肌凝蛋白V（Myosin V）為一雙頭馬達蛋白（motor protein），其沿著肌動蛋白絲（actin filaments）朝正端運動。肌凝蛋白V和其它分子馬達相同，均利用ATP水解及水解產物的釋放而產生機械運動。文中提出一新的多重路徑運動模型，此模型乃基於考慮肌凝蛋白V的兩個馬達區域水解循環各自獨立。

　　在模型中，我們以七個狀態間的轉換來描述肌凝蛋白V 的運動，其中一轉換速率受外力所影響。依此模型，我們可以瞭解肌凝蛋白V依照不同的化學反應路徑完成步進的動作。

　　本文採用酵素動力學計算肌凝蛋白V 的運動速度，並利用隨機過程分析推算其隨機性（randomness）。分析結果顯示，在不同的ATP濃度及外力負載條件下，肌凝蛋白V 會有不同的運動速度、平均運動距離，以及隨機性。由本文理論所推算之值與文獻實驗數據比較，結果相符合。

　　Myosin V is a two-head motor protein that move along actinfilaments toward barbed end. Like the other molecular motors, Myosin V use ATP hydrolysis and product release to produce movement. Here we
propose a new multiple paths model that relies on the independent ATPase activities at the two heads.
　
In this model, the processivity of Myosin V was described by the transition of seven states with one load-dependent rate. According to this model, we can understand how Myosin V step forward by different chemical reaction paths.

　　In this paper, we use the enzyme kinetics and stochastic processes analysis for computing the mean velocity and randomness. The analysis reveal the variation of mean velocity, run length and of
the randomness with ATP concentration under external load. These theoretical results are in agreement with the experimental ones.

1. Mehta, A. D., Rock, R. S., Rief, M., Spudich, J.
A., Mooseker, M. S., and Cheney, R. E. (1999).
Myosin-V is a processive actin-based motor.
Nature 400, 590-593.

2. De La Cruz, E. M., Wells, A. L., Rosenfeld, S.
R., Ostap, E. M., and Sweeney, H. L. (1999). The
kinetic mechanism of myosin V. Proc. Natl. Acad.
Sci. USA 96, 13726-13731.

3. Walker, M. L., Burgess, S. A., Sellers, J. R.,
Wang, F., Hammer , J. Ⅲ A., Trinick, J., and
Knight, P. J. (2000). Two-headed binding of a
processive myosin to F-actin. Nature 405,
804-807.

4. Burgess, S., Walker, M., Wang, F., Sellers, J.
R., White, H. D., Knight, P. J., and Trinick, J.
(2002). The prepower stroke conformation of
myosin V. J. Cell Biol. 159, 983-991.

5. Forkey, J. N., Quinlan, M. E., Shaw, M. A.,
Corrie, J. E., and Goldman, Y. E. (2003).
Three-dimensional structural dynamics of myosin
V by single-molecule fluorescence polarization.
Nature 422, 399-404.

6. Yildiz, A., Forkey, J. N., McKinney, S. A., Ha,
T., Goldman, Y. E., and Selvin, P. R. (2003).
Myosin V walks hand-over-hand: single
fluorophore imaging with 1.5-nm localization.
Science 300, 2061-2065.

7. Coureux, P. D., Wells, A. L., Menetrey, J.,
Yengo, C. M., Morris, C. A., Sweeney, H. L., and
Houdusse, A. (2003). A structural state of the
myosin V motor without bound nucleotide. Nature
425, 419-423.

8. Fuller, G. M. and Shields, D. (2001). Molecular
basis of medical cell biology. McGraw-Kill, New
York.

9. Schliwa, M. (2003). Molecular Motors. Wiley,
Weinheim.

10. Mermall, V., Post, P. L., and Mooseker, M. S.
(1998). Unconventional Myosins in cell
movement, membrane traffic, and signal
transduction. Science 279, 527-533.

11. Reck-Peterson, S. L., Provance, D. W. Jr.,
Mooseker, M. S., and Mercer, J. A. (2000).
Class V myosins. Biochim. Biophys., Acta.
1496, 36-51.

12. Mehta, A. (2001). Myosin learns to walk. J.
Cell Science 114, 1981-1998.

13. Veigel, C., Wang, F., Bartoo, M. L., Sellers,
J. R., and Molloy, J. E. (2002). The gated gait
of the processive molecular motor, myosin V.
Nature Cell Biol. 4, 59-65.

14. Rayment, I., Rypniewski, W. R., Schmidt-Base,
K., Smith, R., Tomchick, D. R., Benning, M.M.,
Winkelmann, D.A., Wesenberg, G., and Holden,
H.M. (1993). Three-dimensional structure of
myosin subfragment-1: a molecular motor.
Science 261, 50-58.

15. Holmes, K. C., Angert, I., Kull, F. J., Jahn,
W., and Schroder, R. R. (2003). Electron
cryo-microscopy shows how strong binding of
myosin to actin releases nucleotide. Nature
425, 423-427.

16. Rief, M., Roc, R. S., Mehta, A. D., Mooseker,
M. S., Cheney, R. E., and Spudich, J. A.
(2000). Myosin-V stepping kinetics: a molecular
model for processivity. Proc. Natl. Acad. Sci.
USA 97, 9482-9486.

17. Baker, J. E., Brosseau, C., Fagnant, P., and
Warshaw, D. M. (2003). The unique properties of
tonic smooth muscle emerge from intrinsic
as well as intermolecular behaviors of Myosin
molecules. J. Biol. Chem. 278, 28533-28539.

18. Yengo, C. M. and Sweeney, H. L. (2004).
Functional role of loop 2 in myosin V.
Biochemistry. Biochemistry 43, 2605-2612.

19. Trybus, K. M., Krementsova, E., and Freyzon, Y.
(1999). Kinetic characterization of a monomeric
unconventional myosin V construct. J. Biol.
Chem. 274, 27448-27456.

20. Wang, M. D., Schnitzer, M. J., Yin, H.,
Landick, R., Gelles, J., and Block, S. M.
(1998). Force and velocity measured for single
molecules of RNA polymerase. Science 282,
902-907.

21. Elston, T. (2000). A macroscopic description of
biomolecular transport. J. Math.Biol.41,189-
206.

22. Gilbert, H. F. (2000). Basic concepts in
biochemistry. McGraw-Hill, New York.

23. Keller, D. and Bustamante, C. (2000). The
mechanochemistry of molecular motors. Biophys.
J. 78, 541-556.

24. Fisher, M. E. and Kolomeisky, A. B. (2001).
Simple mechanochemistry describes the dynamics
of kinesin molecules. Proc. Natl. Acad. Sci.
USA, 98, 7748-7753.

25. Svoboda, K., Mitra, P. P., and Block, S. M.
(1994). Fluctuation analysis of Motor Protein
Movement and Single Enzyme Kinetics. Proc.
Natl. Acad. Sci. USA, 91, 11782-11786.

26. Kolomeisky, A.B.and Fisher, M. E. (2003). A
simple kinetic model describes the processivity
of Myosin-V. Biophys. J. 84, 1642-1650.

27. Sakamoto, T., Amitani, I., Yokota, and E.,Ando,
T. (2000). Direct observation of processive
movement by individual Myosin V molecules.
Biochim. Biophys. Res. Commun. 272, 589-292.

28. Baker, J. E., Krementsova, E. B., Kennedy, G.
G., Armstrong, A., Trybus, K. M., and Warshae,
D. M . (2004). Myosin V processivity: Multiple
kinetic pathways for head-to-head coordination.
Proc. Natl. Acad. Sci. 101, 5542-5546.

29. Visscher, K., Schnitzer, M. J., and Block, S.
M. (1999). Single kinesin molecules studied
with a molecular force clamp. Nature 400,
184-189.