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Fig. 2. Metabolic processes in muscle fibers. The main biochemical processes
involved in energy generation in striated muscle involve the combustion of
fatty acids and carbohydrates. Carbohydrates (orange) are imported
via facilitative processes from the capillary supply lines to the
myofibre, where they may be stored as intramuscular triglycerides or glycogen,
respectively, for later combustion. Fatty acid metabolization (green box) is
an obligatory aerobic process that takes place in mitochondria via
beta-oxidation and the Krebs cycle. In contrast, the `metabolic conversion' of
carbohydrates via glycolysis in the cytoplasm (orange box) is
oxygen-independent and is not necessarily coupled to mitochondrial
respiration. This may lead to the production of the anaerobic end-product
lactate. The decomposition of organic backbones in mitochondria produces
reduction equivalents (and CO2), the former of which drive the
oxygen-dependent generation of ATP via coupling to respiratory chain.
Boxed factors are the crucial proteins involved at successively aligned
transport, storage and conversion steps of metabolic pathways in striated
muscle and whose mRNA expression was investigated. Endothelial LPL is involved
in transporting fatty acids (FA) from the vasculature through the interstitium
into the myocellular compartment (Glatz
and Storch, 2001; Jeukendrup,
2002). There H-FABP is believed to play a main role in the
intramyocellular transport of free FA. HSL liberates free FA from IMCL for
mitochondrial oxidation. CPT I is a key enzyme for the uptake of FA into the
mitochondrial matrix. The Krebs cycle enzymes Fum and SDH and the constituents
of the electron transport chain, NADH6, COX1 and COX4, are then responsible
for oxygen-dependent ATP production during mitochondrial respiration. PFKM
represents a main control step for entry of carbohydrates into the glycolytic
pathway. For further explanation, see List of abbreviations.
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