© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

6.3.4.2 Biological Chemomechanical Power Conversion

Many examples of direct chemomechanical power transduction are found in nature. Perhaps the most familiar is the mechanism of muscle contraction mediated by the sliding of interdigitating myosin and actin filaments -- the actomyosin molecular motor.¹²⁴⁶ Myosin is constructed of domains joined by hinges and is powered by ATP. The globular head domain of the myosin motor, called myosin subfragment-1 or "S1", is a whale-shaped molecular device ~5 nm wide at the head and ~20 nm long, containing ~50,000 atoms (~500 nm³). When the ATP molecule binds to an open ATPase pocket on S1, this action causes a conformational change in the S1 protein, which releases its hold on the associated actin filament. The pocket can then close further, clipping the terminal phosphate from the ATP (making ADP) and powering yet another conformational change which increases affinity for actin. The phosphate is released as S1 again binds to actin, pulling the pocket open wider and triggering a ~2 millisec power stroke as the spent ADP molecule is ejected.⁵⁷⁸ During each 0.05-sec cycle, the myosin motor advances 10 nm along the actin filament (~5 micron/sec velocity) pulling with a force of ~5 pN, a mechanical energy output of ~50 zJ, or ~10^-6 pW (power density ~2 x 10⁶ watts/m³). The energy released in converting one ATP molecule to ADP is ~83 zJ, so the motor is ~60% efficient. A typical macroscale muscle may contain ~10¹¹ individual actomyosin motors. The net energy efficiency of a muscle is at most 25%, doing the greatest work when it shortens by only ~10% of its length.²⁰²² In 1998, the myosin superfamily of protein motors contained 15 different classes, and ~30 different myosin species were known.²²⁷⁹

Other molecular motors similarly transduce chemical energy into mechanical motion to drive the beating of cilia (dynein; Section 9.3.1.1) and to drive the transport of vesicles and organelles along tracks within cells (kinesin; Section 9.4.6). In each case, the binding of ATP (or GTP) induces conformational transitions in the ~12 nm motor proteins, which are reversed by the hydrolysis of bound ATP and release of ADP and phosphate, advancing the motor protein along the tubulin track and pulling with a force of ~2 pN.¹²⁴⁷ Another example is a component of the mitochondrial enzyme ATP synthase: F₁ATPase, a ~1 nm rotor spinning in a ~12 nm barrel, the smallest rotary motor known, producing forces >100 pN and a rotary torque of ~40-80 pN-nm under high load,^1234,3198 and a calculated no-load rotational frequency of ~17 Hz with ~100% energy conversion efficiency.^2278,3198 Another strong molecular motor is RNA polymerase, which pulls with a stall force of ~25 pN at a 12-40% efficiency. Each human tissue cell probably contains ~50 different kinds of biological motors. A different type of chemomechanical motor protein -- an enzyme called lambda exonuclease found in a bacteriophage virus -- chews up one of DNA's double helix strands, leaving behind a single strand; the motor pulls >5 pN while moving along the strand, and is powered directly by the energy liberated from DNA's broken bonds.³¹⁹⁴

In human mitochondria, the combined TCA (tricarboxylic acid) cycle (aka. Krebs cycle⁸⁹²) and cytochrome chain oxidative-phosphorylation convert 39 molecules of ADP to higher-energy ATP during the net metabolism of one glucose molecule, a chemochemical energy conversion efficiency of ~65%.⁵²⁶ Combining this metabolic process with the myosin motor described earlier would yield a glucose-powered chemomechanical transducer that is ~39% efficient.

While all known eukaryotic motors are powered by ATP or GTP, neither is required for bacterial flagellar rotation. Rather, the flagellar ionic motor is a rotary device¹³⁸¹ driven by the pH differential between the cytosolic and extracellular sides of the device. This ~5 x 10⁶ atom, 50,000 nm³ motor consists of a shaft attached to a rotor that spins up to 300 Hz (15 Hz at high load) inside a fixed stator ~30 nm in diameter attached to the peptidoglycan layer, driven by the flow of ~1000 protons (H⁺ ions) per revolution. Stator and rotor surfaces lie ~0.5 nm apart.

Figure 6.2 illustrates how this proton-gradient driven machine works. The rotor has a set of ~100 proton-accepting sites around its periphery which interact with 8 channel complexes embedded in the stator. Each channel complex has two half-channels, one accessible from the (relatively alkaline) cytosol, the other from the (relatively acidic) extracellular space. A proton is transferred from a half-channel to an acceptor site on the rotor, but cannot be donated to the other half-channel unless the rotor rotates.³⁹⁶ Referring to the Figure to trace one cycle, in the geometry shown in (A), site 2 is empty and site 3 is filled, so the complex can move right but not left. If random motion carries the complex one step to the right (B), the proton on site 3 can move into the cytosol and a new proton can move into the adjacent empty site 4 (C). The rotor then rotates counterclockwise a distance equal to the spacing between acceptor sites, to relieve the spring tension (D).⁵⁷⁹ Reversing the geometry (which takes ~1 millisec, controlled by a protein switch) allows the motor to run backwards; this occurs naturally every few seconds in the absence of any environmental stimulus.⁵³¹ The motor develops ~10^-4 pW with a power density of ~2 x 10⁶ watts/m³. Motor efficiency is <5% at low load, 50-99%+ at high load.⁵⁸¹

Flagellar ionic motors that run on sodium ions rather than protons have been found in some bacteria;⁵⁸⁰ no doubt other alternatives could be employed in chemomechanically powered medical nanorobots. Drexler¹⁰ estimates from first principles that chemically-driven engines can be designed to operate at >99% efficiency at a power density of ~10⁹ watts/m³.

Other natural chemomechanical engines involve fluid movements, including evaporative-driven ascent of sap in plants,²⁰³¹ hydrophilic- and hygroscopic-driven expansion due to water absorption, and osmotic pumps in cells, leaves, and plant roots.

Last updated on 18 February 2003