( Enzymes ( ( ( are naturally ) ( occurring ( proteins ( that ( act ( as ( biological catalysts ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Enzymes)) (VP (VBP are) (ADVP (RB naturally)) (VP (VBG occurring) (NP (NP (NNS proteins)) (SBAR (WHNP (WDT that)) (S (VP (VBP act) (PP (IN as) (NP (JJ biological) (NNS catalysts))))))))) (. .)))
Enzymes are naturally occurring proteins that act as biological catalysts.
(ROOT (S (PP (IN In) (NP (DT a) (JJ biological) (NN reaction))) (, ,) (NP (NNS proteins)) (VP (VBP act) (PP (IN as) (NP (NNS catalysts)))) (. .)))