Glyoxylate cycle
The glyoxylate cycle is a metabolic pathway that bypasses carbon-losing steps of the citric acid cycle so cells can turn two-carbon compounds into biosynthetic material.
What the glyoxylate cycle is
The glyoxylate cycle is a variant route connected to the citric acid cycle. It lets cells process acetyl-CoA without losing both carbon atoms as carbon dioxide, which makes it useful when two-carbon compounds are an important carbon source.
The carbon-conserving shortcut
In the ordinary citric acid cycle, two decarboxylation steps release carbon dioxide. The glyoxylate cycle bypasses those steps. Isocitrate lyase splits isocitrate into succinate and glyoxylate, and malate synthase joins glyoxylate with another acetyl-CoA to form malate.
How it connects to the citric acid cycle
The pathway shares several metabolites with the citric acid cycle, including citrate, isocitrate, malate, and oxaloacetate. Its special branch changes the balance of the route: instead of mainly oxidizing acetyl groups for energy, it can preserve carbon skeletons for making sugars and other cell material.
Plants, seeds, and glyoxysomes
In germinating oil-rich seeds, stored fats are broken down into acetyl-CoA. The glyoxylate cycle helps convert that carbon into compounds that can feed gluconeogenesis, supporting early growth before the seedling is fully photosynthetic. In plants, much of this work is associated with specialized peroxisomes called glyoxysomes.
Microbes and acetate growth
Many bacteria and fungi use the glyoxylate cycle when growing on acetate, ethanol, fatty acids, or other substrates that feed metabolism through acetyl-CoA. The pathway gives them a way to build four-carbon intermediates while still drawing energy from available carbon sources.
Regulation and tradeoffs
Cells regulate the pathway because it changes whether isocitrate is sent toward full oxidation or carbon conservation. When energy-rich carbon is plentiful but biosynthetic intermediates are needed, the glyoxylate route can be favored. When rapid energy extraction is the priority, more carbon can flow through the full citric acid cycle.
Why it matters
The glyoxylate cycle helps explain how organisms grow on simple carbon sources and how oil-storing seeds turn fat reserves into new tissue. It also matters in microbiology, plant physiology, metabolic engineering, and the study of pathogens that rely on flexible carbon metabolism inside hosts.