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CMAH
Putative cytidine monophosphate-N-acetylneuraminic acid hydroxylase-like protein is an enzyme that in humans is encoded by the ''CMAH'' gene.〔(【引用サイトリンク】 url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=8418 )〕
== Function ==
Sialic acids are terminal components of the carbohydrate chains of glycoconjugates involved in ligand–receptor, cell–cell, and cell–pathogen interactions. The two most common forms of sialic acid found in mammalian cells are N-acetylneuraminic acid (Neu5Ac) and its hydroxylated derivative, N-glycolylneuraminic acid (Neu5Gc). Studies of sialic acid distribution show that Neu5Gc is not detectable in normal human tissues although it was an abundant sialic acid in other mammals. Neu5Gc is, in actuality, immunogenic in humans.〔
The absence of Neu5Gc in humans is due to a deletion within the human gene CMAH encoding cytidine monophosphate-N-acetylneuraminic acid hydroxylase, an enzyme responsible for Neu5Gc biosynthesis. Sequences encoding the mouse, pig, and chimpanzee hydroxylase enzymes were obtained by cDNA cloning and found to be highly homologous. However, the homologous human cDNA differs from these cDNAs by a 92-bp deletion in the 5' region. This deletion, corresponding to exon 5 of the mouse hydroxylase gene, causes a frameshift mutation and premature termination of the polypeptide chain in human. It seems unlikely that the truncated human hydroxylase mRNA encodes for an active enzyme explaining why Neu5Gc is undetectable in normal human tissues.〔
The deletion that deactivated this gene occurred approximately 3.2 mya, after the divergence of humans from the African great apes, and quickly swept to fixation in the human population. The lineage of this pseudogene in humans indicates another deep split in Africa dating to 2.9 Mya, with a complex subsequent history. Causes of the selection against the CMAH gene could include a severe infectious disease that specifically binds to Neu5Gc, a change in binding preference of a sialic acid binding protein favoring the loss of Neu5Gc or accumulation of Neu5Ac, or protection from viruses originating in individuals with Neu5Gc due to anti-Neu5Gc antibodies in CMAH-negative individuals
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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