Citrullination

Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family. Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family. Enzymes called arginine deiminases (ADIs) catalyze the deimination of free arginine, while protein arginine deiminases or peptidylarginine deiminases (PADs) replace the primary ketimine group (=NH) by a ketone group (=O). Arginine is positively charged at a neutral pH, whereas citrulline has no net charge. This increases the hydrophobicity of the protein, which can lead to changes in protein folding, affecting the structure and function. The immune system can attack citrullinated proteins, leading to autoimmune diseases such as rheumatoid arthritis (RA) and multiple sclerosis (MS). Fibrin and fibrinogen may be favored sites for arginine deimination within rheumatoid joints. Test for presence of anti-citrullinated protein (ACP) antibodies are highly specific (88–96%) for rheumatoid arthritis, about as sensitive as rheumatoid factor (70–78%) for diagnosis of RA, and are detectable from even before the onset of clinical disease. Citrullinated vimentin may be an autoantigen in RA and other autoimmune diseases, and is used to study RA. Moreover, antibodies against mutated citrullinated vimentin (MCV) may be useful for monitoring effects of RA therapy. An ELISA system utilises genetically modified citrullinated vimentin (MCV), a naturally occurring isoform of vimentin to improve the performance of the test. In the reaction from arginine to citrulline, one of the terminal nitrogen atoms of the arginine side chain is replaced by an oxygen. Thus, arginine's positive charge (at physiological pH) is removed, altering the protein's tertiary structure. The reaction uses one water molecule and yields ammonia as a side-product: PADs are found in chordates but not in lower animals. In mammals five PAD isotypes – PAD1, PAD2, PAD3, PAD4 and PAD6 – have been found. PAD5 was thought to be a unique isotype in humans, however it was shown to be homologous to PAD4. These isotypes differ in terms of their tissue and cellular distributions. PAD1 expression has been detected in epidermis and the uterus, and it acts in citrullination of keratin and filaggrin, key components of keratinocytes. PAD2 is expressed at a high level in the central nervous system (CNS), including the eye and brain, as well as skeletal muscle and the spleen. PAD transcripts have been found in the C57BL6/J mouse eyes as early as embryonic day 14.5. PAD2 has also been shown to interact with vimentin in skeletal muscle and macrophages, causing the filaments to disassemble, suggesting a role in apoptosis. One of PAD2's target substrates is myelin basic protein. In the normal retina, deimination is found in nearly all the retinal layers, including the photoreceptors. Deimination has been also reported in neuronal cells, such as astrocytes, microglia and oligodendrocytes, Schwann cells and neurons. Methylation and phosphorylation of MBP are active during the process of myelinogenesis. In early CNS development of the embryo, MBP deimination plays a major role in myelin assembly. In adults, MBP deimination is found in demyelination diseases such as multiple sclerosis. MBP may affect different cell types in each case.