Replace only value in json with javascript

i have this json:
{"agent": "Teste","obs": "obs 2 in other obs","id": 4,"queue": "Suply","name":"Other"}

I need this with Regex
{"agent": "Teste","obs": "<b>obs</b> 2 in other <b>obs</b>","id": 4,"queue": "Suply","name":"Other"}

I fail in replace because the key “obs” is replaced too.