| Fast myosin skeletal heavy chain refers to the fast-twitch myosin heavy chain (MyHC) isoforms that dominate adult type II skeletal muscle fibers and define their rapid contractile kinetics, high shortening velocity, and glycolytic metabolic profile by specifying thick filament composition and cross-bridge cycling properties. The fast MyHC proteins share the canonical myosin II architecture with an N‑terminal globular motor domain that binds and hydrolyzes ATP and interacts with actin, followed by a neck region that associates with essential and regulatory light chains and a long coiled-coil tail that drives dimerization and filament assembly, while isoform-specific sequence differences within the motor and converter regions tune ATPase rate, duty ratio, and actin-sliding velocity. Expression of fast skeletal MyHC isoforms such as MyHC‑IIX and MyHC‑IIA is enriched in fibers that generate high force at short contraction times and rely on glycolytic metabolism, and these isoforms combine with distinct sets of myosin light chains and regulatory proteins to produce fiber-type–specific mechanical outputs that are suited to rapid, phasic movements and high-intensity bursts of activity. During postnatal muscle maturation, embryonic and neonatal MyHC isoforms are progressively replaced by adult fast MyHCs in secondary fibers, and the relative abundance of different fast isoforms is controlled by motor neuron firing patterns, calcium-dependent signaling, and activity-regulated transcriptional programs that adjust muscle performance to functional demand. Regulatory elements in the MyHC gene locus include a fast MyHC super-enhancer identified by single-nucleus ATAC‑seq as a large, open chromatin region that physically interacts with multiple fast MyHC promoters and acts as a central regulatory hub to coordinate high-level, fiber-type–restricted transcription of fast MyHC genes across the locus. Perturbation of this super-enhancer reduces expression of several fast MyHC isoforms, shifts fibers toward slower contractile phenotypes, and alters force–velocity characteristics, indicating that the enhancer architecture of the fast MyHC locus is a key determinant of fiber-type identity and muscle functional specialization. Fast skeletal MyHC expression patterns are widely used as molecular and immunohistochemical markers to classify muscle fiber types, assess plasticity in response to training, unloading, or neuromuscular disease, and evaluate selective fiber atrophy or transformation in myopathies and motor neuron disorders. |
