Karboksylases

Karboksylases are a class of enzymes that catalyze the transfer of a carboxyl group (CO₂) to a substrate, typically in biochemical reactions involving carboxylation. These enzymes play crucial roles in various metabolic pathways, including photosynthesis, fatty acid synthesis, and the citric acid cycle. The carboxylation process often requires cofactors such as biotin, ATP, or magnesium ions, depending on the specific enzyme and reaction mechanism.

One well-known example of a carboxylase is **acetyl-CoA carboxylase**, which catalyzes the conversion of acetyl-CoA to malonyl-CoA in the biosynthesis of fatty acids. This reaction is the first committed step in fatty acid synthesis and involves biotin as a cofactor. Another important carboxylase is **pyruvate carboxylase**, which fixes CO₂ to pyruvate to form oxaloacetate, a key intermediate in gluconeogenesis and the citric acid cycle. This enzyme requires ATP and biotin for activity.

In photosynthesis, **RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)** is the most abundant enzyme on Earth and serves as a carboxylase in the Calvin cycle, where it catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate. However, RuBisCO also exhibits oxygenase activity, leading to photorespiration, a process that competes with carbon fixation.

Karboksylases are classified based on their substrate specificity and cofactor requirements. Biotin-dependent carboxylases, such as acetyl-CoA carboxylase and pyruvate carboxylase, rely on biotin as a prosthetic group to facilitate CO₂ transfer. Other carboxylases, like RuBisCO, do not require biotin but instead utilize metal ions or other cofactors to stabilize reaction intermediates.

The regulation of carboxylases is often tightly controlled to match metabolic demands. For instance, acetyl-CoA carboxylase is subject to allosteric regulation by citrate and palmitoyl-CoA, while pyruvate carboxylase is activated by acetyl-CoA. These regulatory mechanisms ensure efficient carbon flow through metabolic pathways.