( ( Proteins ( that ( act ( as ( catalysts ( in ( biochemical reactions ) ) ) ) ) ) ) ( ( are ( called enzymes ) ) . ) )
(ROOT (S (NP (NP (NNPS Proteins)) (SBAR (WHNP (WDT that)) (S (VP (VBP act) (PP (IN as) (NP (NP (NNS catalysts)) (PP (IN in) (NP (JJ biochemical) (NNS reactions))))))))) (VP (VBP are) (VP (VBN called) (NP (NNS enzymes)))) (. .)))
Proteins that act as catalysts in biochemical reactions are called enzymes.
(ROOT (S (PP (IN In) (NP (DT a) (JJ biological) (NN reaction))) (, ,) (NP (NNS proteins)) (VP (VBP act) (PP (IN as) (NP (NNS catalysts)))) (. .)))