Converts 5-Calpha-glycinylthymidine (Calpha-GlyT) into 5-aminoethyl-2'-deoxyuridine (5-NedU) as a step in the pathway leading to thymidine hypermodifications in the viral genome (PubMed:34522950). As a final result of the pathway of hypermodification, 5-aminoethyl-2'-deoxyuridine (5-NedU) substitutes for about 30% of thymidines in the viral DNA (PubMed:34522950, PubMed:29555775). These modifications probably prevent degradation of viral genome by the host restriction-modification antiviral defense system (PubMed:34522950). 5-C(alpha)-glycyl-dTMP in DNA + H(+) = 5-aminoethyl-dUMP in DNA + CO2 Belongs to the pyridoxal-phosphate-dependent aminodecarboxylase family.