Sialic acids are components of carbohydrate chains of glycoconjugates and are involved in cell-cell recognition and cell-pathogen interactions. Catalyzes the conversion of CMP-N-acetylneuraminic acid (CMP-Neu5Ac) into its hydroxylated derivative CMP-N-glycolylneuraminic acid (CMP-Neu5Gc), a sialic acid abundantly expressed at the surface of many cells. CMP-N-acetyl-beta-neuraminate + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = CMP-N-glycoloyl-beta-neuraminate + 2 Fe(III)-[cytochrome b5] + H2O Binds 1 [2Fe-2S] cluster per subunit. Decreased enzyme activity in presence of 100 mM of NaCl. Amino-sugar metabolism; N-acetylneuraminate metabolism. Belongs to the CMP-Neu5Ac hydroxylase family.