Interconversion of serine and glycine. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O = (6S)-5,6,7,8-tetrahydrofolate + L-serine The kcat/Km for tetrahydrofolate is 0.081 mM(-1)sec(-1). The kcat/Km for L-serine is 0.0022 mM(-1)sec(-1). Optimum pH is 3.0. Stable under acidic conditions. Retains over 70% of its original activity at pH range of 3-6. Optimum temperature is 30 degrees Celsius. One-carbon metabolism; tetrahydrofolate interconversion. Homotetramer. Highest expression in the ovary and testis. 6- to 7-fold lower expression in hemocyte, silk gland, midgut and fat body. Belongs to the SHMT family.