Biochemical processes fall into one of three main categories: metabolic pathways, signal communication networks and gene regulatory mechanisms. Metabolic pathways may be of the anabolic or catabolic variety, responsible for the synthesis and degradation of biological molecules. Signal transduction refers to the molecular reactions involved in transmission of a signal from the cell membrane to the cell's interior to evoke the desired modification in the cell's activity. The expression of genes is controlled by a regulatory mechanism that senses and responds to chemical environments within cells.
Proteins play a major role in almost all biochemical processes. Metabolic pathways require enzymes to catalyze several steps of chemical reactions. These enzymes are protein in nature and act as catalysts by lowering the activation energy of reactions. Signal transduction mechanisms also involve proteins that are bound to the surface of cell membranes in the form of receptors, or proteins present in the cellular cytoplasm. A change in the location of the protein or a modification of its activity leads to signal transmission across and through the cell. Gene expression is regulated through the binding of transcription factors with regulatory sites on the DNA. These transcription factors are also protein in nature.
All biochemical processes in the body need energy to drive them forward. The compound adenosine triphosphate (ATP) serves as the major energy currency of the body. Other high energy compounds such as creatine phosphate also provide energy through the cleavage of the high-energy phosphoester bonds. However, energy consumption is kept to a minimum through the phenomenon of coupling of reactions. Certain reactions that require energy are linked to others that release energy.
A complex system of mechanisms governs the activity levels of all biochemical processes. This mainly involves the feedback system in which presence of particular levels of a certain biomolecule inhibits some part of the process. For example, the presence of surplus amounts of the product of a metabolic pathway inhibits the action of the enzymes that catalyze the rate-limiting step of the reaction.