Enzymatic Catalysis

Maintaining life in any cell requires hundreds of chemical reactions to take place at any given moment, and in a highly regulated manner. These reactions allow the cell to grow or divide, produce energy, decompose waste products, communicate with other cells, and more. Most of the reactions happening in the average cell have to do with the metabolic activity of the cell, which can be grouped into two types. Catabolic reactions degrade and oxidize foodstuff material (carbohydrates, fats, and sometimes proteins) in order to extract the chemical energy stored in it. This is done very gradually, using numerous highly regulated reactions that are organized as pathways. In the presence of oxygen, foodstuff is completely oxidized to carbon dioxide (CO₂), with the release of energy in the form of electrons. For example, in the catabolism of glucose, the parent molecule is first degraded partially by glycolysis, a 10-step catabolic pathway well known to students of biochemistry^[1]. The product of this pathway, pyruvate, is then activated and fully oxidized to carbon dioxide by another well-known pathway, the citric acid (Krebs) cycle ^[2]. The high-energy electrons produced by these pathways are first temporarily stored on the electron carriers NADH and FADH ₂, and are then converted by highly complex cellular machinery into a chemically more stable form of energy, ATP. This molecule is nicknamed the ‘universal currency’, since it is ubiquitous in cells and can be easily used to drive energy-demanding cellular processes. There are many such processes, most of which are attributed to the other type of metabolic reaction, termed anabolic. Indeed, anabolism includes all chemical reactions used by cells to build complex materials; these reactions use energy formed during catabolic reactions. In some cases, anabolic pathways also use a reducing agent, in the form of NADPH. That is because complex cellular molecules are often reduced. A well-known anabolic pathway is gluconeogenesis, which builds glucose from lactic acid and glycerol derivatives.