ABSTRACT
The two metalloproteins comprising nitrogenase are more often called Protein I and Protein II. Protein I contains the cofactor with iron and molybdenum called the FeMo-co and it is here that the molecular nitrogen is reduced to ammonia. Protein I is a tetramer of about 220,000 Da formed of two a-subunits and two /J-subunits with similar molecular masses. The total metallic composition of Protein I is about 2 Mo, 30 Fe and 30 S (Eady, 1986; Newton, 1993). Recent crystallisation of Protein I of A. vinelandi and C. pasteu'rianum has helped in elucidating its three-dimensional structure as well as that of the associated prosthetic groups, as shown in Fig. 9.2 (Kim and Rees, 1992; Dean et a\., 1993). The FeMo-co comprise the 4 Fe : 3S group linked to a group IMo : 3Fe : 3S. One of the essential constituents of FeMoco is homocitrate, which is directly bound to an atom of molybdenum. Two FeMo-co groups are bound to an a-subunit. Four molecules of the other prosthetic group called T clusters' are bound covalently to the cysteine residues of the a-and /?-subunits. Protein II is a dimer of about 68,000 Da formed of two identical subunits connected by a unique 4 Fe: 4 S prostethic group (Eady, 1986; Newton, 1993; Dean et al., 1993). Protein II transfers an electron to Protein I in the presence of ATP.
