The primary structure of subunit II of NADH dehydrogenase from bovine-heart mitochondria

Subunit II (with a molecular mass of about 24000 dalton, approximately 24 kDA) of NADH dehydrogenase from beef heart mitochondria was [ 14C ]carboxymethylated and cleaved with CNBr and proteolytic enzymes. Sequence analyses of purified fragments suggest that the subunit is composed of a homogeneous polypeptide chain, containing just over 230 residues. The primary structure of this chain was established except for a 14-residue internal part which was only determined by composition. The amino acid sequence suggests that four cysteine residues are involved in the binding of an iron-sulfur cluster. The subunit contains no long hydrophobic segment, in contrast to structures often found in membrane proteins, but in agreement with a model where the functional unit of NADH dehydrogenase in the membrane is shielded by other intra-membrane proteins. The polypeptide has a weak similarity to the iron-sulfur binding region of ferredoxin and has interesting but possibly insignificant similarities to parts of previously compared flavin-linked enzymes.