NAD-dependent L-serine dehydrogenase that catalyzes the oxidation of L-serine and methyl-L-serine and is possibly involved in serine catabolism. Has low activity toward beta-hydroxyisobutyrate. L-serine + NAD(+) = aminoacetaldehyde + CO2 + NADH kcat is 10.4 sec(-1) with L-serine as substrate. kcat is 9.6 sec(-1) with methyl-DL-serine as substrate. kcat is 5.8 sec(-1) with DL-glycerate as substrate. kcat is 11.6 sec(-1) with methyl-2,2-dimethyl-3-hydroxypropionate as substrate. kcat is 1.6 sec(-1) with NAD as substrate. Amino-acid degradation. Homotetramer, dimer of dimers. Belongs to the HIBADH-related family.