The correlation between DNA methylation and chromatin structure, as it relates to transcriptional activity, is demonstrated by the observation that there are several proteins, that bind to methylated CpGs but not to unmethylated CpGs, whose functions are integrated into transcriptional regulation. There are currently 15 genes in the human genome that encode proteins that bind to methyl-CpG in DNA. These 15 proteins are divided into 3 subfamilies identified by structural similarities. These sub-families are the methyl binding domain (MBD) proteins, the methyl-CpG-binding zinc finger proteins (also called the Kaiso family), and the SRA domain ( S ET and R ING finger domain A ssociated) containing proteins. The SET domain is so-called because it was first identified in three Drosophila proteins called S uppressor of variegation variant 3-9 [Su(var)3-9], E nhancer of zeste, and T rithorax. The RING domain is a zinc-finger-like domain which gets its name from the term R eally I nteresting N ew G ene.
Metabolism of propanoate begins with its conversion to propionyl coenzyme A (propionyl-CoA), the usual first step in the metabolism of carboxylic acids. Since propanoic acid has three carbons, propionyl-CoA can directly enter neither beta oxidation nor the citric acid cycles. In most vertebrates, propionyl-CoA is carboxylated to D-methylmalonyl-CoA, which is isomerised to L-methylmalonyl-CoA. A vitamin B 12 -dependent enzyme catalyzes rearrangement of L-methylmalonyl-CoA to succinyl-CoA, which is an intermediate of the citric acid cycle and can be readily incorporated there.