Required for the solubility and assembly of the heterotetramer 3-ketoacyl-[acyl carrier protein] (ACP) reductase functional complex (KAR or KAR1) that forms part of the mitochondrial fatty acid synthase (mtFAS). Alpha-subunit of the KAR complex, acts as scaffold protein, required for the stability of carbonyl reductase type-4 (CBR4, beta-subunit of the KAR complex) and for its 3-ketoacyl-ACP reductase activity, thereby participating in mitochondrial fatty acid biosynthesis. Catalyzes the NAD-dependent conversion of (3R)-3-hydroxyacyl-CoA into 3-ketoacyl-CoA (3-oxoacyl-CoA) with no chain length preference, this enzymatic activity is not needed for the KAR function. Prefers (3R)-3-hydroxyacyl-CoA over (3S)-3-hydroxyacyl-CoA and displays enzymatic activity only in the presence of NAD(+)(H). Cooperates with enoyl-CoA hydratase 1 in mitochondria, together they constitute an alternative route to the auxiliary enzyme pathways for the breakdown of Z-PUFA (cis polyunsaturated fatty acid) enoyl-esters (By similarity). NAD-dependent 17-beta-hydroxysteroid dehydrogenase with highest activity towards estradiol. It efficiently catalyzes the oxidation of estradiol (E2), testosterone, and dihydrotestosterone. Primarily an oxidative enzyme, it can switch to a reductive mode determined in the appropriate physiologic milieu and catalyze the reduction of estrone (E1) to form biologically active estradiol (E2) (PubMed:9712896). a (3R)-3-hydroxyacyl-CoA + NAD(+) = a 3-oxoacyl-CoA + H(+) + NADH 17beta-estradiol + NAD(+) = estrone + H(+) + NADH NAD(+) + testosterone = androst-4-ene-3,17-dione + H(+) + NADH 17beta-hydroxy-5alpha-androstan-3-one + NAD(+) = 5alpha-androstan-3,17-dione + H(+) + NADH Lipid metabolism; fatty acid biosynthesis. Steroid biosynthesis; estrogen biosynthesis. Lipid metabolism; mitochondrial fatty acid beta-oxidation. Heterotetramer with CBR4; contains two molecules of HSD17B8 and CBR4. Kidney, liver, testis, ovary and spleen (PubMed:9712896, PubMed:15923359). Oviduct, uterus, mammary gland, vagina, prostate, clitoral gland and moderately heart, dorsal skin, brain and lung (PubMed:15923359). Belongs to the short-chain dehydrogenases/reductases (SDR) family.