Page 111 - Algae Anatomy, Biochemistry, and Biotechnology

94 Algae: Anatomy, Biochemistry, and Biotechnology

etc. all of which need energy input. We know that such changes can occur in proteins, the most
likely molecules serving these locomotory functions in real movement system. But what drives
and controls these changes? In principle, the problem is not difficult. Altering the ionic milieu,
changing chemically or electrically its environment can alter the tertiary or quaternary structure
of a protein. In most control systems, if not all, a change in the environment brings about a
change in the properties of the motor, acting either directly or indirectly on the component of
the motor. We need only two proteins to make a motor using the sliding filaments mechanism,
that is, a globular protein (such as tubulin) and an anchor protein (such as the dynein–dynactin
complex). If the globular protein can polymerize, we can assemble it into a linear polymer that
can be attached via the anchor protein to another structure some distance away. The transformation
of chemical energy into mechanical work depends on a conformational change of the anchor
protein, which uses the hydrolysis of ATP into ADP. Provided the anchor protein repeats the con-
formational change upon each monomer of the globular protein in turn, the “boat” can be hauled
“hand over hand” towards the distant anchorage. Provided some kind of metachrony regulates adja-
cent motor molecules, we can link our small movements in a temporal series to amplify the amount
of movement that can be achieved. Each step costs hydrolysis of one ATP molecule per anchor
protein. The simplest and most obvious solution is either to have more than one anchor protein,
or to have a dimer, working out-of-phase, being careful not to detach before the new attachment
is formed. For instance, most (but not all) microtubular motors (dyneins, kinesins) work as
dimers whose subunits walk along microtubule walls just like human legs walk on a surface.
Once we have two hands to pull on the rope we can indeed move hand-over-hand; the one-
armed man cannot do more than pull once. The flagellum movements are due to the transient inter-
action between two anchored microtubules, coupled to a linkage control. The generation of sliding
of adjacent doublets by flagellar dynein is combined to the resisting forces localized near the active
sliding rows of dyneins. During the cycle of binding/release obtained by dynein conformational
change coupled to ATP hydrolysis, chemical energy is converted into mechanical energy used
for sliding. Owing to their regular spacing every 24 nm along the axoneme, several adjacent
dyneins participate to this local sliding and their functioning proceeds by local waves that propagate
step by step all the way along the flagella. The postulated regulator has therefore to trigger the func-
tioning of the different dyneins alternatively along the length as well as around the section of the
axoneme, but its molecular nature remains unknown.
The model of Lindemann (1994) accounts for wave generation and propagation, regulated by
geometrical constraints. This model, the so-called “geometric clutch,” is based on the way a cylin-
der with nine generatrix (the nine outer doublets of the axoneme) changes form when submitted to
bending. In the zone of curvature, the doublets located outside the curvature are brought apart while
the doublets located inside the curvature come closer to each other. This makes the corresponding
dynein molecules efficient for sliding. In contrast, dynein molecules located outside the curvature
are too far for binding to adjacent microtubules: no active sliding can occur in this zone. This model
is consistent with ultrastructural data of electron microscopy. As above, the functioning of the
axonemal mechanics needs transient connections, which could possibly be disrupted by intrinsic
proteolytic activities due to the combined activities of a protease/ligase system.

INTERNAL FLAGELLAR STRUCTURE
How a Paraflagellum Rod Works

As the PFR and the axoneme are very tightly connected, both structures have to move together. It
can be argued that the PFR contributes to regulate the flagellar movements. This could be done by
providing a more rigid structure that can vary its stiffness in time, that is, modifying flagellum
beating pattern.