Characterization of inhibitory effects of NH 2 OH and its N-methyl derivatives on the O 2 -evolving complex of Photosystem II

Inorganic cofactors (Mn, Ca 2+ and Cl - ) are essential for oxidation of H 2 O to O 2 by Photosystem II. The Mn reductants NH 2 OH and its N-methyl derivatives have been employed as probes to further examine the interactions between these species and Mn at the active site of H 2 O oxidation. Results of these studies show that the size of a hydroxylamine derivative regulates its ability to inactivate O 2 evolution activity, and that this size-dependent inhibition behavior arises from the protein structure of Photosystem II. A set of anions (Cl - , F - and SO 4 2- ) is able to slow NH 2 OH and CH 3 NHOH inactivation of intact Photosystem II membranes by exerting a stabilizing influence on the extrinsic 23 and 17 kDa polypeptides. In contrast to this non-specific anion effect, only Cl - is capable of attenuating CH 3 NHOH and (CH 3 ) 2 NOH inhibition in salt-washed preparations lacking the 23 and 17 kDa polypeptides. However, Cl - fails to protect against NH 2 OH inhibition in salt-washed membranes. These results indicate that the attack by NH 2 OH and its N-methyl derivatives on Mn occurs at different sites in the O 2 -evolving complex. The small reductant NH 2 OH acts at a Cl - -insensitive site whereas the inhibitions by CH 3 NHOH and (CH 3 ) 2 NOH involve a site that is Cl - sensitive. These findings are consistent with earlier studies showing that the size of primary amines controls the Cl - sensitivity of their binding to Mn in the O 2 -evolving complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43537/1/11120_2004_Article_BF00046773.pd

Micronutrient needs of tropical food crops

The trace element requirements of tropical crops are reviewed with special reference to their role, the differences between crop species, the differences between cultivars, paths of trace element uptake, effects of climate, the importance of crop management and yield levels