The alpha-helix, an overlooked molecular motor

摘要

At first sight the alpha-helix appears as a rigid scaffold braced by hydrogen bonds nearly parallel to the helix axis. Looked at more closely it turned out to be highly dynamic and able to transform chemical into mechanical energy. The hydrogen bonds are fairly weak and compliant bonds. Their length, usually between 0.267 and 0.291 nm ( mean value, 0.28 nm), depends on the interaction of the side chains. The most important strong interaction is the electrostatic repelling force between equally charged side chains ( Glu(-), Asp(-), Lys(-), Arg(-)), well known by experiments with polyamino acids. In proteins with different amino acids, repelling forces between charged side chains work in the axial direction and stretch the hydrogen bonds. Extreme shortening of the hydrogen bonds occurs when ions, e. g., Ca2+, H+, or PO3-, are added and discharge side chains. This means a cooperative pitch decrease of the alpha-helix ( pitch range between 0.52 and more than 0.55 nm; mean value, 0.54 nm). This pitch change is absolutely connected by steric reasons with torque generation and torsional rotations, as demonstrated by molecular and tubular alpha-helix models. Thus, charged alpha-helices are molecular motors propelled by the electrostatic energy of added ions. The motor effect is most striking with highly charged alpha-helical coiled coils, e. g., tropomyosin, myosin, and alpha-actinin that can rotate actin filaments by winding and unwinding. For example, the shortening of muscle depends on the sliding ( drilling) motion of the Ca2+-activated helical actin filaments into the cross-bridges of the A-band. Here, models are presented for the in vitro sliding of actin filaments and for cytoplasmic streaming by winding and unwinding of myosin chains, and for membrane proteins that contain nonhelical domains between membrane-penetrating alpha-helices. They may transport molecules by the described torsional rotations if they perform supercoiling. Winding and supercoiling can lead to displacement of bound ions and to a feedback-regulated oscillation between two different coiling stages E-1 and E-2 that explain "eversion". The models need the torque for 1-2 rotations. They explain active and passive transports, the driving-effects of ion gradients, ATP hydrolysis by unwinding, ATP synthesis by winding up of the supercoils, etc.