Update viperin.md

content/3.defense-systems/viperin.md

@@ -25,7 +25,7 @@ Recently,  Viperin-like enzymes were found in prokaryotes (pVips).  Strikingly
 
 ## Molecular mechanism
 
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+!Figure1](/viperin/human_vip.jpg){max-width=750px}
 Fig.1: Catalytic activity of human Viperin generates ddhCTP :ref{doi=10.1002/1873-3468.13778}
 
 Viperins are members of the radical S-adenosylmethionine (rSAM) superfamily. This group of enzymes use a [4Fe-4S] cluster to cleave S-adenosylmethionine (SAM) reductively, generating a radical which is generally transferred to a substrate. It was demonstrated that through their [4Fe-4S] cluster catalytic activity, eukaryotic viperins convert a ribonucleotide, the cytidine triphosphate (CTP) into a modified ribonucleotide, the 3'-deoxy-3',4'-didehydro-CTP (ddhCTP) :ref{doi=10.1038/s41586-018-0238-4},(5).