Glycolysis is a metabolic pathway occurring in the cytoplasm of nearly all living cells, which breaks down a single six-carbon glucose molecule to extract energy. It represents the initial stage of cellular respiration, the process cells use to convert biochemical energy into adenosine triphosphate (ATP). Although glycolysis can proceed without oxygen, it is the universal first step for both anaerobic and aerobic respiration.
The Two Major Phases of Glycolysis
The glycolytic pathway comprises a sequence of ten enzyme-catalyzed reactions divided into two distinct phases: the preparatory phase and the payoff phase. The first half of the pathway is the energy investment phase, as it consumes energy to prepare the glucose molecule for cleavage. This initial stage modifies the stable glucose ring, making it ready to be split. The entire process for both phases takes place in the cell’s cytosol.
The preparatory phase begins when an enzyme called hexokinase uses one ATP molecule to add a phosphate group to glucose, forming glucose-6-phosphate. This initial phosphorylation traps the glucose molecule inside the cell and destabilizes its structure. After an isomerization reaction that rearranges its atoms, a second ATP molecule is invested. The enzyme phosphofructokinase-1 (PFK-1) adds another phosphate group, creating a molecule ready to be divided.
This six-carbon molecule is then split by the enzyme aldolase into two different three-carbon sugar phosphates. One of these products can proceed directly into the next phase, while the other is converted into the correct form by another enzyme.
The second half of glycolysis is the payoff phase, where the cell harvests the energy contained in the two three-carbon molecules generated earlier. This phase is characterized by the production of energy-rich molecules. Because the preparatory phase created two three-carbon sugars, the reactions in the payoff phase occur twice for every glucose molecule, allowing for a net gain of energy.
Each three-carbon sugar is oxidized, and its energy is captured as high-energy electron carriers and ATP. Electrons are transferred to the coenzyme NAD+, forming NADH. Subsequently, through a series of four more reactions, chemical rearrangements occur to facilitate the production of ATP. The final product of this sequence is pyruvate, a three-carbon molecule.
Net Products and Energy Yield
The process begins with a single molecule of glucose as the primary input. Along with glucose, two molecules of ATP are invested during the preparatory phase, and two molecules of the coenzyme NAD+ are used as electron acceptors.
The outputs from the breakdown of one glucose molecule are two molecules of pyruvate, four molecules of ATP, and two molecules of NADH. Two water molecules are also generated as byproducts during these transformations.
While four ATP molecules are generated in the payoff phase, two were consumed in the preparatory phase, resulting in a net gain of two ATP molecules. Additionally, the cell gains two molecules of NADH, which can be used later in other metabolic processes to generate more ATP. Although the yield of two ATP per glucose is modest, glycolysis is an extremely rapid process that can supply a quick burst of energy.
Regulation of the Glycolytic Pathway
The rate of the glycolytic pathway is not constant; it is regulated to match the cell’s fluctuating energy demands. This control is achieved by modulating the activity of specific enzymes, which ensures the cell produces ATP when energy is low and conserves its glucose resources when energy is abundant.
The primary mechanism for controlling glycolysis is feedback inhibition, where the products of the pathway signal to slow down the initial steps. High concentrations of ATP, a direct product of glycolysis, act as an allosteric inhibitor. This means ATP binds to specific regulatory sites on enzymes, separate from their active sites, changing their shape and reducing their activity when the cell has sufficient energy.
Three enzymes serve as the main control points: hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase. PFK-1 is inhibited by high levels of ATP but is activated by high levels of AMP (adenosine monophosphate), an indicator of a low energy state. This dual sensitivity allows PFK-1 to act like a switch, turning the pathway on or off based on the cell’s energy needs. Hormonal signals, such as insulin, can also influence glycolysis by promoting the synthesis of these regulatory enzymes.
The Fates of Pyruvate
The fate of this three-carbon molecule depends on the availability of oxygen within the cell. This presence or absence of oxygen determines which metabolic pathway pyruvate will enter next, placing glycolysis at a junction in cellular energy metabolism.
In aerobic conditions, pyruvate is transported from the cytoplasm into the mitochondria. There, it undergoes a process called oxidative decarboxylation, converting it into a two-carbon molecule called acetyl-CoA. This acetyl-CoA then enters the citric acid cycle (also known as the Krebs cycle), where it is completely oxidized to carbon dioxide. This process generates a significant amount of additional ATP through oxidative phosphorylation.
Conversely, in anaerobic conditions where oxygen is scarce, cells rely on fermentation. The primary purpose of fermentation is not to produce more ATP directly, but to regenerate the NAD+ that was converted to NADH during glycolysis, which is necessary for the pathway to continue. In human muscle cells during strenuous exercise, pyruvate is converted into lactate. In yeast, it undergoes alcoholic fermentation to produce ethanol and carbon dioxide.