The decarboxylase enzyme, also called carboxy-lyases, is an enzyme that catalyzes the decarboxylation of amino acids, beta-keto acids and alpha-keto acids. Carboxy-lyases are carbon-carbon lyases that add or remove a carboxyl group from organic compounds. They are categorized under EC number 4.1.1.
Decarboxylases have a variety of roles in metabolic pathways and carbohydrate synthesis. There are many types of decarboxylases, which perform different functions. They are usually named after the substrate whose decarboxylation they catalyze. Some notable carboxy-lyases include ornithine decarboxylase, which is involved in the urea cycle and L-aromatic amino-acid decarboxylase, which helps to convert L-dopa to dopamine and 5-HTP to serotonin.
Decarboxylase Function and Mechanism
The ornithine decarboxylase enzyme catalyzes the decarboxylation of ornithine, which is a product of the urea cycle. This is the committed step in polyamine synthesis, including in polyamines such as putrescine, spermidine and spermine. The protein has 461 amino acids in humans, and the active form also forms a homodimer. It is essential for cell growth and produces the polyamines that are needed to stabilize newly synthesized DNA.
L-aromatic amino-acid decarboxylase (AADC) catalyzes a number of different decarboxylation reactions. It converts L-dopa to dopamine, 5-HTP to serotonin, L-Histidine to histamine, and L-Tryptophan to tryptamine, among other functions. The enzyme uses the active form of vitamin B6, pyridoxal phosphate, as a cofactor. Some reactions produced by AADC probably don't have much biological significance or even any at all. For example, it is the enzyme histidine decarboxylase that is responsible for biosynthesizing histamine in the human body and in other animals too.
Glutamate decarboxylase (GAD) is another example of a notable carboxy-lyase, which has two isoforms - GAD67 and GAD65. The enzyme catalyzes the decarboxylation of glutamate to GABA and carbon dioxide and uses PLP as a cofactor. It is expressed in the brain and GABA is used as a neurotransmitter, as well as in insulin-producing cells in the pancreas. The two GAD isoforms fulfill different roles and are found in different locations in the cell. They are both present in all synapses in the human nervous system.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (known as Rubisco, rubisco, RuBPCase, or RuBPco) is the only carboxy-lyase that leads to a net fixation of carbon dioxide. The enzyme is involved in the first major step of carbon fixation when plants and other photosynthetic organisms convert carbon dioxide to energy-rich molecules. It catalyzes the carboxylation of ribulose-1,5-bisphosphate, and there is likely no other enzyme on Earth that is as abundant.
Various other types of decarboxylase exist and perform different functions to catalyze the decarboxylation of different amino acids, beta-keto acids and alpha-keto acids. Some of them don't seem to have much of a real role, while others play an important part in different mechanisms and processes in both the human body and other organisms. They are also coded by different genes and can have various effects if there is a deficiency or overabundance. This category of enzymes covers many different things, including enzymes that have a broad range of characteristics, reactions and functions.