The Primary Structure of Subunit II of NADH Dehydrogenase from Bovine‐Heart Mitochondria

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Abstract
Subunit II (with a molecular mass of about 24000 dalton, ∼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 part of previously compared flavin‐linked enzymes.