Overview of Myosin Heavy Chain

What is Myosin Heavy Chain (MHC)?

Myosin Heavy Chain (MHC) is a large polypeptide chain that, together with myosin light chains, forms the myosin molecule. This molecule is a key player in the contractile mechanisms of muscle cells. The heavy chain consists of several domains that contribute to its function as a molecular motor, converting chemical energy from ATP hydrolysis into mechanical work.

Structure of Myosin Heavy Chain

General Structure

The MHC is a pivotal component of the myosin molecule, which functions as a molecular motor responsible for muscle contraction and various cellular movements. The MHC itself is a large polypeptide chain, typically ranging from 200 to 250 kDa in molecular weight, and is composed of three main regions: the head, neck, and tail.

• Head Region: The head region, also known as the motor domain, is critical for its ATPase activity and actin-binding capabilities. This region is where the energy from ATP hydrolysis is converted into mechanical force, allowing the myosin molecule to "walk" along actin filaments.
• Neck Region: The neck region acts as a lever arm that amplifies the small conformational changes occurring in the head region. This amplification is crucial for efficient force generation and movement. The neck is also the binding site for myosin light chains (MLCs), which modulate the motor function of MHC.
• Tail Region: The tail region is responsible for the dimerization of two myosin heavy chains, facilitating the formation of the myosin molecule's coiled-coil structure. This region also plays a role in the assembly of thick filaments in muscle fibers, contributing to the structural integrity and organization of muscle tissues.

Functional Domains

Each functional domain within the MHC contributes to its role as a molecular motor:

ATPase Activity in the Head Region: The head region contains an ATPase site that hydrolyzes ATP, providing the necessary energy for the conformational changes required for movement. The ATP hydrolysis cycle involves several key steps:

• ATP Binding: The myosin head binds ATP, causing a conformational change that reduces its affinity for actin, allowing it to detach from the actin filament.
• ATP Hydrolysis: ATP is hydrolyzed to ADP and inorganic phosphate (Pi), which are retained on the myosin head, leading to another conformational change that repositions the head into a "cocked" state.
• Weak Binding to Actin: The myosin head weakly binds to a new position on the actin filament.
• Power Stroke: Release of Pi initiates the power stroke, a conformational change that generates force and moves the actin filament. ADP is then released, and the myosin head remains tightly bound to actin until a new ATP molecule binds.

Lever Arm in the Neck Region: The neck region, acting as a lever arm, amplifies the conformational changes in the head region, translating small movements into larger displacements of the actin filament. This amplification is essential for efficient muscle contraction and cellular movement.

Tail Region Functions: The tail region's coiled-coil structure enables the dimerization of two myosin heavy chains, forming a stable myosin molecule. In muscle cells, the tail regions of multiple myosin molecules associate to form thick filaments, which are essential for the contractile apparatus. This structural organization allows for the coordinated interaction of myosin with actin filaments during muscle contraction.

Types of MHC Isoforms

Skeletal Muscle Isoforms

Skeletal muscles contain multiple MHC isoforms, which are primarily categorized based on their expression in different muscle fiber types:

MHC I (Type I Fibers): Also known as slow-twitch fibers, Type I fibers are rich in MHC I isoforms. These fibers are designed for endurance and continuous, low-intensity activities. They have a high capacity for aerobic metabolism, characterized by high mitochondrial content and capillary density, which supports prolonged contractions without fatigue.

MHC IIa (Type IIa Fibers): Type IIa fibers, or fast-twitch oxidative fibers, express MHC IIa isoforms. These fibers combine high speed and power with a moderate resistance to fatigue. They are capable of both aerobic and anaerobic metabolism, making them versatile for a variety of physical activities.

MHC IIx (Type IIx Fibers): Also known as fast-twitch glycolytic fibers, Type IIx fibers contain MHC IIx isoforms. These fibers are designed for short bursts of power and speed. They have lower mitochondrial content compared to Type I and IIa fibers and rely heavily on anaerobic glycolysis, leading to rapid fatigue.

MHC IIb (Type IIb Fibers): Found predominantly in small mammals but less so in humans, Type IIb fibers express MHC IIb isoforms. These fibers are the fastest and most powerful, used for explosive movements. They fatigue quickly due to their reliance on anaerobic metabolism.

MHC class I and class II and their role as mediators during antigen presentation and recognition (Deeg et al., 2014)

Cardiac Muscle Isoforms

The heart muscle expresses two primary MHC isoforms, which are crucial for its continuous and rhythmic contractions:

Alpha MHC (α-MHC): Predominantly found in the atria and to a lesser extent in the ventricles of the heart. Alpha MHC is associated with high ATPase activity, enabling rapid and efficient contractions, which are essential for the swift pumping action of the atria and the high-output requirements of the ventricles under certain conditions.

Beta MHC (β-MHC): The primary isoform in the ventricular muscle. Beta MHC has lower ATPase activity compared to alpha MHC, resulting in slower, more energy-efficient contractions. This isoform is crucial for maintaining sustained, forceful contractions over long periods, which is essential for the heart's endurance and consistent performance.

Schematic illustration of the cardiac β-myosin heavy chain (β-MHC) (Aboonabi et al., 2024).

Differences Between Alpha and Beta Myosin Heavy Chains

Non-Muscle Isoforms

Non-muscle cells express MHC isoforms that are essential for various cellular functions beyond muscle contraction:

Non-Muscle Myosin II-A, II-B, and II-C: These isoforms are involved in cell motility, cytokinesis, and maintaining cell shape. They play crucial roles in processes such as wound healing, immune response, and development.

Genetic and Molecular Basis of MHC Isoforms

The diversity of MHC isoforms is underpinned by a complex genetic and regulatory framework:

Gene Family: The MHC isoforms are encoded by a family of genes located on different chromosomes. In humans, these genes include MYH7 (β-MHC), MYH6 (α-MHC), MYH1 (IIx MHC), MYH2 (IIa MHC), and MYH3 (embryonic MHC), among others. Each gene has unique regulatory elements that control its expression in specific muscle types and developmental stages.

Regulation of Expression: The expression of MHC isoforms is tightly regulated by various factors, including developmental signals, hormonal influences, and mechanical stress. For instance, thyroid hormone levels significantly influence the expression of α-MHC and β-MHC in cardiac muscle. Mechanical loading and physical activity can also shift the expression profile of MHC isoforms in skeletal muscle, a phenomenon known as muscle plasticity.

Developmental Aspects: During development, the expression of MHC isoforms undergoes a well-orchestrated sequence. Embryonic and neonatal isoforms, such as MYH3 and MYH8, are expressed during early stages and are gradually replaced by adult isoforms like MYH7 and MYH2 as the muscle matures.

Functional Implications of MHC Isoforms

The specific properties of each MHC isoform are tailored to the functional requirements of the muscle fiber types they are expressed in:

Contractile Speed and Force: Isoforms with higher ATPase activity, such as α-MHC and MHC IIb, enable rapid and powerful contractions, suitable for high-intensity activities. In contrast, isoforms with lower ATPase activity, like β-MHC and MHC I, support slower, more sustainable contractions, ideal for endurance.

Metabolic Efficiency: The metabolic properties of muscle fibers are closely linked to the MHC isoforms they express. Slow-twitch fibers with MHC I isoforms have high oxidative capacities, making them efficient for prolonged aerobic activities. Fast-twitch fibers with MHC IIx or IIb isoforms rely more on glycolytic metabolism, suitable for quick, intense bursts of activity.

What Do Myosin Heavy Chains Do?

Mechanism of Muscle Contraction

Sliding Filament Theory

The sliding filament theory explains how muscles contract to produce force. According to this theory, muscle contraction occurs when the thick (myosin) and thin (actin) filaments slide past each other, shortening the overall length of the muscle fiber. The MHC plays a crucial role in this process by interacting with actin filaments through a cyclical process known as the cross-bridge cycle.

Cross-Bridge Cycle

The cross-bridge cycle is a series of molecular events that result in muscle contraction. Here are the key steps involved:

• Binding of Myosin to Actin: In the resting state, the myosin head (part of the MHC) is in a "cocked" position, bound to ADP and inorganic phosphate (Pi). When the myosin-binding sites on actin are exposed (due to the action of calcium ions binding to troponin, causing a shift in tropomyosin), the myosin head binds to actin, forming a cross-bridge.
• Power Stroke: The release of Pi triggers the power stroke, where the myosin head pivots, pulling the actin filament towards the center of the sarcomere. This action generates force and shortens the muscle fiber. During the power stroke, ADP is released from the myosin head.
• Detachment: A new ATP molecule binds to the myosin head, causing it to detach from the actin filament. This detachment is necessary for the next cycle of contraction.
• Re-cocking: The myosin head hydrolyzes the newly bound ATP into ADP and Pi, returning to the cocked position and ready to bind to another actin site, initiating another cycle of contraction.

This cyclical process continues as long as ATP and calcium ions are available, allowing for sustained muscle contraction.

Force Generation and Muscle Performance

Different MHC isoforms are tailored to the specific functional demands of various muscle fibers, influencing their contractile properties and performance.

Correlation Between MHC Isoforms and Muscle Fiber Types

Slow-Twitch Fibers (Type I): These fibers contain MHC I isoforms, which are adapted for endurance activities. They generate less force but can sustain contractions for prolonged periods without fatigue. These fibers are rich in mitochondria and have a high oxidative capacity, making them efficient for aerobic metabolism.

Fast-Twitch Fibers (Type IIa, IIx, IIb): These fibers express different fast MHC isoforms (IIa, IIx, and IIb), each contributing to high-speed, powerful contractions. Type IIa fibers (MHC IIa) have both high force production and moderate endurance, making them versatile for various physical activities. Type IIx fibers (MHC IIx) generate greater force and speed but fatigue quickly. Type IIb fibers (predominantly in small mammals) are specialized for explosive movements, producing the highest force but also the quickest fatigue.

Impact on Muscle Strength, Speed, and Endurance

Muscle Strength: The strength of a muscle is largely determined by the type and amount of MHC isoforms present. Muscles with a higher proportion of fast-twitch fibers (Type II) can generate greater force compared to those with a predominance of slow-twitch fibers (Type I).

Muscle Speed: The speed of muscle contraction is influenced by the ATPase activity of the MHC isoforms. Fast MHC isoforms (e.g., MHC IIb) have higher ATPase activity, enabling rapid ATP hydrolysis and quicker cross-bridge cycling, resulting in faster muscle contractions.

Muscle Endurance: Endurance is associated with the muscle's ability to sustain contractions over time. Slow-twitch fibers (Type I), with MHC I isoforms, are more resistant to fatigue due to their efficient aerobic metabolism and high oxidative capacity. They are better suited for activities requiring prolonged muscle use, such as long-distance running or maintaining posture.

Broader Implications in Cellular Function

Beyond their role in muscle contraction, MHCs are involved in various cellular processes critical for cell motility, division, and intracellular transport.

Cell Motility

MHCs are key players in cell motility, enabling cells to move and navigate their environment. This is particularly important in processes such as:

• Wound Healing: Cells move to the site of injury to repair tissue damage.
• Immune Response: Immune cells migrate towards pathogens to mount an effective response.
• Development: Cellular movements are crucial during embryonic development for proper tissue and organ formation.

Cytokinesis

During cell division, MHCs contribute to cytokinesis, the process where the cytoplasm of a parent cell is divided to form two daughter cells. Non-muscle myosin II, a specific isoform, forms a contractile ring at the cleavage furrow, helping to pinch the cell into two separate entities.

Intracellular Transport

MHCs are involved in the transport of vesicles, organelles, and other cargo within cells. This intracellular transport is essential for maintaining cellular organization and function, particularly in large, complex cells like neurons.

Differences Between Light Chain and Heavy Chain Proteins

Myosin light chains (MLCs) and myosin heavy chains (MHCs) are both crucial components of the myosin motor protein complex, yet they differ significantly in their structure, function, and roles in muscle and cellular activity. The following table outlines the key differences between these two types of proteins:

Myosin Heavy Chains Analysis Methods

The analysis of myosin heavy chains (MHCs) is crucial for understanding their structure, function, and role in muscle physiology and diseases. Various advanced methods are employed to study MHCs, each offering unique insights and capabilities.

Mass Spectrometry

Mass spectrometry (MS) is a powerful technique for analyzing the molecular composition of proteins, providing detailed information about their structure and modifications.

Tandem Mass Spectrometry (MS/MS)

Tandem mass spectrometry involves the digestion of proteins into peptides, which are then ionized and fragmented. The resulting mass spectra reveal the peptide sequences, allowing for the identification and characterization of MHC isoforms. MS/MS is highly sensitive and accurate, making it ideal for in-depth analysis of MHCs, including the detection of post-translational modifications (PTMs) and protein-protein interactions. This technique is essential for understanding the complex nature of MHCs and their functional implications in muscle biology.

MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization-Time of Flight)

MALDI-TOF mass spectrometry uses a laser to ionize proteins or peptides, which are then accelerated through a time-of-flight analyzer. This separates ions based on their mass-to-charge ratio, allowing for rapid identification and characterization of MHC isoforms and their modifications. MALDI-TOF is known for its speed and high-throughput capabilities, making it suitable for analyzing large proteins like MHCs efficiently.

Chromatography

Chromatography is another vital technique for the purification, separation, and analysis of proteins, including MHCs.

High-Performance Liquid Chromatography (HPLC)

HPLC separates proteins based on their interactions with the stationary phase and their solubility in the mobile phase. This method is highly effective for purifying and quantifying MHC isoforms from complex protein mixtures. HPLC offers high resolution and reproducibility, making it a reliable technique for analyzing the specific MHC isoforms present in muscle tissues.

Affinity Chromatography

Affinity chromatography leverages specific binding interactions between proteins and ligands attached to a chromatography matrix. This method is particularly useful for purifying MHC isoforms using antibodies or other specific binding molecules. Affinity chromatography is highly specific and efficient, enabling the isolation of target MHC proteins from complex samples with high purity.

References

1. Deeg, Janosch A. Modulation of T cell Activation with Nano-and Micronanopatterned Antigen Arrays. Diss. 2014.
2. Aboonabi, Anahita, and Mark D. McCauley. "Myofilament dysfunction in diastolic heart failure." Heart Failure Reviews 29.1 (2024): 79-93.

For research use only, not intended for any clinical use.