Systems Level Regulation of Cardiac Energy Fluxes Via Metabolic Cycles: Role of Creatine, Phosphotransfer Pathways, and AMPK Signaling - Systems Biology of Metabolic and Signaling Networks - page 283

domain theoretically provides four binding sites for adenylates [referred to as sites
1-4 (Kemp et al. 2007 ; Hardie et al. 2011 )], but only sites 1, 3, and 4 can be
occupied in the mammalian enzyme, while site 2 is nonfunctional. The precise role
of these sites is still unclear. Initial evidence suggested that site 4 binds AMP tightly
in a non-exchangeable manner, while site 1 is a high-affinity site for AMP involved
in allosteric activation (see below) and site 3 represents a lower affinity site
(binding AMP, ADP, and ATP) more involved in regulating

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-Thr172 phosphory-
lation (Xiao et al. 2007 ). A more recent study suggests that also site 4 can bind ATP
(and causes site 3 to be empty) and that both sites 3 and 4 may play a role in
allosteric activation (Chen et al. 2012 ). The γ 2- and γ 3-isoforms contain N-terminal
extensions that are subject to truncation by RNA splicing and whose molecular
structure and function are currently unknown. Mutations in the CBS domains of the
AMPK

2 subunit, expressed at particularly high levels in heart, cause the
Wolff-Parkinson-White (WPW) syndrome, a hereditary cardiomyopathy of vary-
ing severity, involving cardiac hypertrophy, contractile dysfunction, and
arrhythmias. Mutations impair adenylate binding and thus AMPK activation
(Scott et al. 2004 ; Burwinkel et al. 2005 ), but the major cause for the cardiomyopa-
thy is the increased AMP-independent basal AMPK activity. This leads to higher
glucose uptake, accumulation of glycogen in cardiac myocytes, and finally
impairment of normal heart muscle development (Burwinkel et al. 2005 ; Davies
et al. 2006 ).

γ

11.5.4 Network Connectivity: AMPK Input Signals and
Upstream Regulation

AMPK integrates various intra- and extracellular signals and maintains cross talk
with other signaling pathways, all related to the cellular energy and nutrient state.
This makes the kinase a central signaling hub in sensing and regulating cellular
energetics and ATP-dependent functions. Indeed, most recent research revealed
that AMPK activation is much more complex than initially anticipated and depends
on multiple covalent modifications and allosteric effectors (Fig. 11.13 ). AMPK
regulation also evolved from a more simple state as, e.g., in the yeast AMPK
homologues that lack allosteric activation by AMP (see below) to the more complex
regulation present in mammals.

11.5.4.1 Covalent Regulation by Phosphorylation

The phosphorylation state at -Thr172 defines the primary activation of AMPK.

This is determined by the balance of different upstream kinases and phosphatases.
There are potentially three mammalian AMPK upstream kinases, with the major
one in most cell types, including heart, being Liver Kinase B1 (LKB1, also called

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