Metabolic Dissipative Structures - Systems Biology of Metabolic and Signaling Networks - page 179

A second kind of temporal metabolic structure is given by circadian rhythms.
With a period close to 24 h (given by the Earth's rotation) circadian clocks enable
cells to adapt their metabolism to the appropriate time of the day, synchronizing
timing of metabolic reactions with cyclic changes in the external environment
(Schibler and Sassone-Corsi 2002 ; Schibler and Naef 2005 ; Dunlap et al. 2004 ;
Wijnen and Young 2006 ). Circadian rhythms govern a wide variety of metabolic
and physiological processes in all organisms from prokaryotes to human cells
(Wijnen and Young 2006 ). In some cells, at least 10 % of all cellular transcripts
oscillate in a circadian manner (Nakahata et al. 2007 ), and in certain cells it has
been observed that between 80 and 90 % of the transcripts seem to follow a pattern
of circadian expression with a period of 24-26 h (Connor and Gracey 2011 ).
A third kind of molecular rhythms is given by spatial concentration waves.
When spatial inhomogeneity elicits instabilities in the intracellular medium,
propagating concentration waves can be triggered. This dynamic behavior is not
only closely related to metabolic oscillations but also to synchronization. Biochem-
ical waves are quite common and involve several cellular variables such as intra-
cellular pH, membrane potential, flavoproteins, calcium, and NAD(P)H . They are
linked to central metabolic processes and specific physiological functions, namely,
signal transduction and intercellular communication (Petty 2006 ). There are several
types of molecular waves and they vary in their chemical composition, velocity,
shape, intensity, and location (Scemes and Giaume 2006 ; Galas et al. 2000 ; Guthrie
et al. 1999 ). Examples of metabolic waves are Na + and Ca 2+ (Bernardinelli
et al. 2004 ), redox (Romashko et al. 1998 ), reactive oxygen species (Aon
et al. 2004 ; Zhou et al. 2010 ), ATP (Ueda et al. 1990 ; Newman 2001 ), pH (Petty
et al. 2000 ), NAD(P)H (Petty and Kindzelskii 2001 ), NAD(P)H coupled with
calcium (Slaby and Lebiedz 2009 ), actin filament assembly during cell locomotion
(Vicker 2002 ), and phosphatidylinositol

(3,4,5)-trisphosphate (PIP 3 ) (Asano

et al. 2008 ).
Metabolic rhythms constitute one of the most genuine properties of multi-
enzymatic dynamics. The conditions required for the emergence and sustainability
of these rhythms, and how they are regulated, represent a biological problem of the
highest significance. However, in spite of its physiological importance many
aspects of these spatiotemporal structures, such as their relationship to the cell
cycle, are still poorly understood and thus deserve further attention.

8.1.3 Dissipative Multienzymatic Complexes:
Metabolic Subsystems

Multienzymatic associations can be viewed as dissipative structures in which
molecular rhythms and functional integrative processes can emerge increasing the
efficiency and control of the catalytic reactions involved (De la Fuente 2010 ;Dela
Fuente and Cortes 2012 ). Self-organization and self-assembly processes allow for