Many cellular processes require energy. Most of these reactions are enzymatically coupled to another energy liberating reaction. Metabolic energy sources, like sugars and fats are broken down into smaller, more useful species. Cells contain a number of high energy molecules to use as sources for these reactions.
The most common of these is adensosine triphosphate, ATP. Cleaving one phosphate group from this liberates 7.3 kcal/mole.
Nicotinamide Adenine Dinucleotide
Nicotinamide adenine dinucleotide, NADH, plays a central role in oxidative metabolism. Through the mitochondrial electron transport chain, NADH can transfer two electrons and a hydrogen ion to oxygen, liberating 52.6 kcal/mole. This is enough energy to synthesize 7.2 ATPs from ADP and Pi. Some inefficiency, though, allows only 3 ATPs to be formed.
||Flavin Adenine Dinucleotide
Another important part of oxidative metabolism is flavin adenine dinucleotide, FADH2. Oxidation to FAD releases 43.4 kcal/mole, enough to generate nearly six ATPs. Again, some inefficiency in electron transport results in the formation of only two ATPs.
Muscle cells use this phosphorylated form of creatine to store energy. Normal metabolism can not produce energy as quickly as a muscle cell can use it, so an extra storage source is needed. The phosphate group can be quickly transferred to ADP to regenerate the ATP necessary for muscle contraction. Hydrolysis of creatine phosphate to creatine releases 10.3 kcal/mole.