Metabolic Probing of Sialylated Glycoconjugates with Fluorine-Selenol Displacement Reaction (FSeDR)

Dysregulated sialic acid biosynthesis is characteristic of the onset and progression of human diseases including hormone-sensitive prostate cancer and breast cancer. The sialylated glycoconjugates involved in this process are therefore important targets for identification and functional studies. To date, one of the most common strategies is metabolic glycoengineering, which utilizes N-acetylmannosamine (ManNAc) analogues such as N-azidoacetylmannosamine (ManNAz) to hijack sialic acid biosynthesis and label the sialylated glycoconjugates with "click chemistry (CuAAC)" tags. Yet, current chemical modifications including those CuAAC-based alkyne/azide tags are still big in size, and the resulting steric hindrance perturbs the mannosamine and sialic acid derivatives' recognition and metabolism by enzymes involved in biosynthetic pathways. As a result, the peracetylated ManNAz has compromised incorporation to sialic acid substrates and manifests cellular growth inhibition and cytotoxicity. Herein, we show that the α-fluorinated peracetylated analogue ManN(F-Ac) displayed a satisfying safety profile in mammalian cell lines at concentrations as high as 500 μM. More importantly, aliphatic selenol-containing probes can efficiently displace α-fluorine in fluoroacetamide-containing substrates including ManN(F-Ac) at a neutral pH range (∼7.2). The combined use of peracetylated ManN(F-Ac) and the dethiobiotin-selenol probe as the fluorine-selenol displacement reaction (FSeDR) toolkit allowed for successful metabolic labeling of sialoglycoproteins in multiple prostate and cancer cell lines, including PC-3 and MDA-MB-231. More sialoglycoproteins in these cell lines were demonstrated to be labeled by FSeDR compared with the traditional CuAAC approach. Lastly, with FSeDR-mediated metabolic labeling, we were able to probe the cellular expression level and spatial distribution of sialylated glycoconjugates during the progression of these hormone-sensitive cancer cells. Taken together, the promising results suggest the potential of the FSeDR strategy to efficiently and systematically identify and study sialic acid substrates and potentially empower metabolic engineering on a diverse set of glycosylated proteins that are vital for human diseases.