ULTIMATE COMPUTING - Quantum Consciousness Studies

90 Cytoskeleton/Cytocomputer
certain critical concentration (Cc) of tubulin, microtubules increase in relation to
the total tubulin concentration. However, MT assembly is more complex than
being simply at equilibrium with a pool of unassembled subunits.
Spontaneous assembly of tubulin dimers depends on physiological conditions
which alter the critical concentration (Cc) of subunits required. Spontaneous MT
assembly thus depends on many cofactors: calcium ion concentration,
temperature, presence of microtubule inhibitors or stabilizers (Figure 5.8),
microtubule associated proteins (Figure 5.12), the presence of microtubule
organizing centers (MTOC-Figure 5.9) and the availability of GTP. Like ATP
(adenosine triphosphate-required for actin assembly), GTP (guanosine
triphosphate) is a source of biochemical energy. As will be described in Chapter
6, ATP and GTP can each donate phosphate bond energy by being “hydrolyzed”
to their “diphosphates” ADP and GDP, respectively. In the case of assembly of
both MT and actin filaments, the presence of GTP or ATP without being
hydrolyzed is an important requirement for assembly. Nonhydrolyzable analogs of
GTP are equally effective in promoting assembly of MT; hydrolysis and energy
input take place after incorporation of a dimer into a tubule. MT formed with
nonhydrolyzable analogs are more stable than those with GTP, so hydrolysis
energy may be related to disassembly. In actin filaments, hydrolysis also occurs
after the formation of the polymer. Therefore a paradox exists in that GTP
binding is required for microtubule assembly, but GTP hydrolysis occurs later and
is not required for assembly. An identical situation occurs with actin and ATP.
Depletion of ATP (for actin) and GTP (for MT) strongly inhibits disassembly but
does not hamper assembly per se. However, in energy depleted cells, random
as8embly prevails instead of organized assembly. The energy from phosphate bond
hydrolysis, the main energy currency in all biological systems, is unaccounted for
within the cytoskeleton, the dynamic organizer of cell function. Perhaps the
energy is used to generate communicative lattice vibrations, coherent excitations,
or “solitons” in MT and the cytoskeleton in general (Chapters 6 and 8).