Catabolic and anabolic pathways are two main classifications for most metabolic pathways. Catabolic pathways break down complex molecules into a few simple products, producing ATP. On the other hand, anabolic pathways synthesize complex end products from simple precursors, requiring ATP hydrolysis. Regulatory signals, like intracellular allosteric activators or inhibitors, can influence the rate of a metabolic pathway. Metabolism integration is achieved through intercellular signaling, primarily through chemical signaling involving hormones or neurotransmitters. Chemical signals, such as hormone or neurotransmitter binding, are transduced by second messenger molecules to activate intracellular responders. Adenylyl cyclase (AC), located on the cell membrane, responds to chemical signals like glucagon and epinephrine by synthesizing cyclic adenosine monophosphate (cAMP). The binding of a hormone to its cell-surface G protein-coupled receptor activates a guanosine triphosphate-dependent regulatory protein (G protein), which subsequently activates AC. The resulting cAMP activates protein kinase A, leading to the phosphorylation of various enzymes, causing their activation or deactivation. Phosphorylation can be reversed by phosphatases. Cells with mitochondria and sufficient oxygen supply engage in aerobic glycolysis, where pyruvate is produced as the end product.
Most pathways can be classified as either catabolic (degrade complex molecules to a few simple products with ATP production) or anabolic (synthesize complex end products from simple precursors with ATP hydrolysis). The rate of a metabolic pathway can respond to regulatory signals such as intracellular allosteric activators or inhibitors. Intercellular signaling provides for the integration of metabolism. The primary route of this communication is chemical signaling (for example, by hormones or neurotransmitters). Second messenger molecules transduce a chemical signal (hormone or neurotransmitter binding) to appropriate intracellular responders. Adenylyl cyclase (AC) is a cell membrane enzyme that synthesizes cyclic adenosine monophosphate (cAMP) in response to chemical signals, such as the hormones glucagon and epinephrine. Following binding of a hormone to its cell-surface G protein–coupled receptor, a guanosine triphosphate–dependent regulatory protein (G protein) is activated that, in turn, activates AC. The cAMP produced activates protein kinase A, which phosphorylates a variety of enzymes, causing their activation or deactivation. Phosphorylation is reversed by phosphatases. Aerobic glycolysis, in which pyruvate is the end product, occurs in cells with mitochondria and an adequate supply of oxygen ([O2],