An allylic ketyl radical intermediate in clostridial amino-acid fermentation

The human pathogenic bacterium Clostridium difficile thrives by the fermentation of l-leucine to ammonia, CO(2), 3-methylbutanoate and 4-methylpentanoate under anaerobic conditions. The reductive branch to 4-methylpentanoate proceeds by means of the dehydration of (R)-2-hydroxy-4-methylpentanoyl-CoA to 4-methylpent-2-enoyl-CoA, which is chemically the most demanding step. Ketyl radicals have been proposed to mediate this reaction catalysed by an iron-sulphur-cluster-containing dehydratase, which requires activation by ATP-dependent electron transfer from a second iron-sulphur protein functionally similar to the iron protein of nitrogenase. Here we identify a kinetically competent product-related allylic ketyl radical bound to the enzyme by electron paramagnetic resonance spectroscopy employing isotope-labelled (R)-2-hydroxy-4-methylpentanoyl-CoA species. We also found that the enzyme generated the stabilized pentadienoyl ketyl radical from the substrate analogue 2-hydroxypent-4-enoyl-CoA, supporting the proposed mechanism. Our results imply that also other 2-hydroxyacyl-CoA dehydratases and the related benzoyl-CoA reductases-present in anaerobically living bacteria-employ ketyl radical intermediates. The absence of radical generators such as coenzyme B12, S-adenosylmethionine or oxygen makes these enzymes unprecedented in biochemistry.     

阿其克苏河床胡杨林长势及影响因素分析

文章采取地面实地观测分析与遥感分析相结合,在研究区域内,对选择的样地进行林分测量并对森林生长状况进行了动态监测,通过对立木状况测量分析等方法进行长势状况分析。结果表明该地区胡杨林趋于衰败,面临着毁灭的危险。气候和地下水位的变化不是导致胡杨死亡的直接原因。     

结合代数的表示理论基础：第1卷：表示理论的技术

一个复杂的数学对象通过另一个较简单的对象表示出来，是一个基本的数学思想，它在分类问题中特别有用。本书从箭图及同调代数的线性表示的观点给出代数闭域上有限维结合代数的表示理论的现代结果。全书分两卷，本书为第1卷，论述表示理论的基本技术，如箭图理论技术、殆分裂序列、标题理论、整二次型等，其中不少材料第一次由原始论文改写为专著的形式。[第一段]