Serine can be degraded to pyruvate via three different pathways:

via direct dehydratation of serine to pyruvate, catalyzed by
serine dehydratase

via serinolysis with glycerate 2-P and phosphoenolpyruvate as
intermediates (catalyzing enzymes: serine pyruvate transaminase,
glycerate dehydrogenase and glycerate kinase).

via cystathionine and cysteine as intermediates (catalyzing enzymes: cystathionine β-synthetase, gamma cystathionase, cysteine desulfhydrase).

serine degradation (animation)

References

Serinolysis: main page

Serine: main page