Activated SAMHD1, an HIV-Fighting Enzyme (image) Johns Hopkins Medicine Caption Four identical subunits make up the antiviral SAMHD1 enzyme. Each subunit must bind a molecule of GTP at its A1 site and an A, T, C or G nucleotide at its A2 site before the enzyme can break down additional A, T, C and G nucleotides at its S sites. Credit Stivers Lab, Johns Hopkins Medicine Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.