Myosin head
{{Short description|Part of myofilaments}}
{{Infobox protein family
| Symbol = Myosin_head
| Name = Myosin_head
| image = PDB 1kk7 EBI.jpg
| width =
| caption = Scallop myosin in the near-rigor conformation
| Pfam = PF00063
| Pfam_clan = CL0023
| InterPro = IPR001609
| SMART =
| PROSITE = PDOC00017
| MEROPS =
| SCOP = 1mys
| TCDB =
| OPM family =
| OPM protein =
| CAZy =
| CDD = cd00124
}}
The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a head, neck, and tail domain; the myosin head binds to thin filamentous actin, and uses ATP hydrolysis to generate force and "walk" along the thin filament. Myosin exists as a hexamer of two heavy chains,{{cite journal | vauthors = Hayashida M, Maita T, Matsuda G | title = The primary structure of skeletal muscle myosin heavy chain: I. Sequence of the amino-terminal 23 kDa fragment | journal = J. Biochem. | volume = 110 | issue = 1 | pages = 54–9 |date=July 1991 | pmid = 1939027 | doi = 10.1093/oxfordjournals.jbchem.a123543}} two alkali light chains, and two regulatory light chains. The heavy chain can be subdivided into the globular head at the N-terminal and the coiled-coil rod-like tail at the C-terminal, although some forms have a globular region in their C-terminal.
There are many cell-specific isoforms of myosin heavy chains, coded for by a multi-gene family.{{cite journal | vauthors = Eller M, Stedman HH, Sylvester JE, Fertels SH, Wu QL, Raychowdhury MK, Rubinstein NA, Kelly AM, Sarkar S | title = Human embryonic myosin heavy chain cDNA. Interspecies sequence conservation of the myosin rod, chromosomal locus and isoform specific transcription of the gene | journal = FEBS Lett. | volume = 256 | issue = 1–2 | pages = 21–8 |date=October 1989 | pmid = 2806546 | doi = 10.1016/0014-5793(89)81710-7| s2cid = 12047829 | doi-access = free }} Myosin interacts with actin to convert chemical energy, in the form of ATP, to mechanical energy.{{cite journal | vauthors = Warrick HM, De Lozanne A, Leinwand LA, Spudich JA | title = Conserved protein domains in a myosin heavy chain gene from Dictyostelium discoideum | journal = Proc. Natl. Acad. Sci. U.S.A. |volume = 83 | issue = 24 | pages = 9433–7 |date=December 1986 | pmid = 3540939 | pmc = 387152 | doi = 10.1073/pnas.83.24.9433| bibcode = 1986PNAS...83.9433W | doi-access = free }} The 3-D structure of the head portion of myosin has been determined {{cite journal | vauthors = Rayment I, Rypniewski WR, Schmidt-Bäse K, Smith R, Tomchick DR, Benning MM, Winkelmann DA, Wesenberg G, Holden HM | title = Three-dimensional structure of myosin subfragment-1: a molecular motor | journal = Science | volume = 261 | issue = 5117 | pages = 50–8 |date=July 1993 | pmid = 8316857 | doi = 10.1126/science.8316857| bibcode = 1993Sci...261...50R }} and a model for the actin-myosin complex has been constructed.{{cite journal | vauthors = Rayment I, Holden HM, Whittaker M, Yohn CB, Lorenz M, Holmes KC, Milligan RA | title = Structure of the actin-myosin complex and its implications for muscle contraction | journal = Science | volume = 261 | issue = 5117 | pages = 58–65 |date=July 1993 | pmid = 8316858 | doi = 10.1126/science.8316858| bibcode = 1993Sci...261...58R }}
The globular head is well conserved,{{cite journal | vauthors = Molloy JE, Burns JE, Kendrick-Jones J, Tregear RT, White DC | title = Movement and force produced by a single myosin head | journal = Nature | volume = 378 | issue = 6553 | pages = 209–12 |date=November 1995 | pmid = 7477328 | doi = 10.1038/378209a0 | bibcode = 1995Natur.378..209M | s2cid = 4334476 }}{{cite journal | vauthors = Lewalle A, Steffen W, Stevenson O, Ouyang Z, Sleep J | title = Single-molecule measurement of the stiffness of the rigor myosin head | journal = Biophysical Journal | volume = 94 | issue = 6 | pages = 2160–9 |date=March 2008 | pmid = 18065470 | pmc = 2257899 | doi = 10.1529/biophysj.107.119396 | bibcode = 2008BpJ....94.2160L }} and is key to contraction. Muscle contraction results from an attachment–detachment cycle between the myosin heads extending from myosin filaments and the sites on actin filaments. The myosin head first attaches to actin together with the products of ATP hydrolysis, performs a power stroke associated with release of hydrolysis products, and detaches from actin upon binding with new ATP. The detached myosin head then hydrolyses ATP, and performs a recovery stroke to restore its initial position. The strokes have been suggested to result from rotation of the lever arm domain around the converter domain, while the catalytic domain remains rigid.{{cite journal | vauthors = Minoda H, Okabe T, Inayoshi Y, Miyakawa T, Miyauchi Y, Tanokura M, Katayama E, Wakabayashi T, Akimoto T, Sugi H | title = Electron microscopic evidence for the myosin head lever arm mechanism in hydrated myosin filaments using the gas environmental chamber | journal = Biochemical and Biophysical Research Communications | volume = 405 | issue = 4 | pages = 651–6 |date=February 2011 | pmid = 21281603 | doi = 10.1016/j.bbrc.2011.01.087 }}