S-Adenosylhomocysteinase from yellow lupine seeds: stoichiometry and reactions of the enzyme.cntdot.adenosine complex

Plant (Lupinus luteus) S-adenosylhomocysteinase, an alpha 2 dimer, forms a 1:2 enzyme-adenosine complex. The binding sites for adenosine are not equivalent. Binding of the first molecule of adenosine is fast (k greater than 7 x 10(5) M-1 s-1), whereas the second molecule of adenosine binds in a slow process with a half-life of 5 min. Adenosine in the 1:1 and 1:2 enzyme--substrate complexes reacts slowly (k = 0.05 min-1) to give finally free enzyme, adenine, and ribose. The enzyme does not lose its ability to catalyze the synthesis of S-adenosylhomocysteine during the reactions. The relevance of the data to the catalytic functioning of the plant S-adenosylhomocysteinase is discussed.