3C proteinase enzyme from hepatitis A virus, molecular model. This proteinase (protein- cleaving) enzyme is produced by the hepatitis A virus. This enzyme facilitates the replication of the virus once it has entered the cell. Hepatitis A virus enters the body through the gut and causes acute inflammation of the liver. Symptoms include jaundice, vomiting and fever. In this model, the two subunits of the enzyme can be clearly distinguished. Solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices; arrows are beta sheets), and the faded tubular structure behind represents the individual atoms and bonds.