Glyceraldehyde 3-phosphate dehydrogenase, known as GAPDH, is an enzyme found in nearly all living organisms, from bacteria to humans. This protein is fundamental for cellular existence, performing a wide array of functions within cells. Its most recognized role involves the production of cellular energy.
The Primary Role in Energy Production
GAPDH performs a specific step in glycolysis, a metabolic pathway that breaks down glucose to generate energy for the cell. Glycolysis takes place in the cell’s cytoplasm and is a fundamental process for producing adenosine triphosphate (ATP), the primary energy currency of cells. The pathway also produces molecules that can be used in other metabolic processes.
During glycolysis, GAPDH catalyzes the conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate. This reaction involves the addition of a phosphate group and the reduction of nicotinamide adenine dinucleotide (NAD+) to NADH. GAPDH ensures that the sugar molecule is correctly modified to continue its journey toward energy generation.
A Standard for Measurement in Scientific Research
GAPDH is widely recognized in scientific research as a “housekeeping protein” because it is expressed at relatively stable levels across most cell types and conditions. Housekeeping proteins are necessary for basic cellular functions and are consistently produced by cells. This consistent expression makes GAPDH a useful tool for researchers.
Scientists often use GAPDH as a “loading control” in experiments, particularly in techniques like Western blotting or quantitative polymerase chain reaction (qPCR). When comparing protein or gene expression levels between different samples, researchers need to ensure that they have loaded equal amounts of starting material into each lane or well. Using GAPDH as a loading control helps confirm that the amounts of ingredients are consistent across all batches.
Observing a consistent GAPDH signal, such as a band of expected intensity in a Western blot or a stable signal in qPCR, indicates that comparable amounts of sample were used. This allows researchers to confidently attribute any observed differences in their protein of interest to experimental conditions rather than to variations in sample loading or preparation. This practice helps ensure the reliability and reproducibility of experimental results.
The “Moonlighting” Functions of GAPDH
Beyond its established role in glycolysis, GAPDH is also known as a “moonlighting protein,” meaning it performs other, distinct functions within the cell that are unrelated to its primary metabolic job.
GAPDH has been observed to participate in various non-glycolytic processes, including the initiation of programmed cell death, known as apoptosis. It also plays a part in DNA repair mechanisms, helping to maintain the integrity of the cell’s genetic material. Furthermore, GAPDH can be involved in viral replication, assisting certain viruses in their life cycles within host cells. These varied roles highlight the surprising adaptability of this single enzyme.
When the Housekeeper is Unreliable
While GAPDH is often considered a stable reference for scientific experiments, its amount can vary under certain physiological or pathological conditions. This can impact its reliability as a loading control. For instance, in some types of cancer, GAPDH expression can be unusually elevated, and its levels may also change in cells experiencing low oxygen conditions, known as hypoxia.
GAPDH is also directly implicated in the progression of some neurodegenerative diseases, such as Alzheimer’s and Huntington’s diseases. In these conditions, GAPDH can malfunction or interact abnormally with other proteins, contributing to cellular dysfunction and disease pathology. Researchers must exercise caution and validate GAPDH’s stability for each specific experimental context, as its “housekeeping” status can be compromised.