Synthase

Synthase is a generic term for an enzyme that catalyzes a synthesis reaction, typically joining two molecular substrates to form a larger product. The name is used widely in biochemistry as a suffix for enzyme names, such as citrate synthase, fatty acid synthase, and sucrose synthase. The use of the suffix does not imply a single mechanism; synthases may or may not require energy input from ATP or other nucleoside triphosphates.

In many metabolic pathways, synthases catalyze condensation or polymerization reactions, often operating on carbon–carbon or carbon–heteroatom bonds. Citrate synthase in the citric acid cycle combines oxaloacetate and acetyl-CoA to form citrate without direct ATP hydrolysis. Other enzymes called synthases, such as sucrose synthase, catalyze reversible synthesis and cleavage of disaccharides and may couple reaction direction to substrate availability. Some synthases function as part of large multi-enzyme complexes, such as fatty acid synthase, which extends carbon chains using malonyl units and reducing equivalents supplied by NADPH.

Distinction from synthetases and ligases: Historically, enzymes that use energy to form bonds were called synthetases, while ligases join molecules using ATP or other energy sources. The term synthase itself does not strictly indicate energy requirements; modern classifications focus on EC numbers and reaction types. Nevertheless, in current usage, “synthase” remains a common name for enzymes that catalyze bond-forming synthesis steps.