In mammals, there are different metabolic pathways in cells that break down fuel molecules to transfer their energy into high energy compounds such as adenosine-5′-triphosphate (ATP), guanosine-5′-triphosphate (GTP), reduced nicotinamide adenine dinucleotide (NADH2), reduced flavin adenine dinucleotide (FADH2) and reduced nicotinamide adenine dinucleotide phosphate (NADPH2).