Catalyzes activation of 4-hydroxyphenylacetate decarboxylase under anaerobic conditions by generation of an organic free radical on a glycine residue, via a homolytic cleavage of S-adenosyl-L-methionine (SAM). glycyl-[protein] + reduced [flavodoxin] + S-adenosyl-L-methionine = 5'-deoxyadenosine + glycin-2-yl radical-[protein] + H(+) + L-methionine + semiquinone [flavodoxin] Binds 2 [4Fe-4S] clusters. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Monomer. Belongs to the organic radical-activating enzymes family.