氨基酸合成

硝酸盐和亚硝酸盐还原酶的辅基:(a)硝酸盐还原酶的钼辅 因子，钼的载体为喋啉衍生物。(b)是卟啉的衍生物， 羧 基可以作为氢离子的供体，将亚硝酸盐还原为铵盐。
硝酸盐同化酶结 构域的序列构成
打破3个共价 键需要大量 的能量
从固氮酶发现的两 种金属串: (a) P串,2个Fe4S3串共用 第四个硫，并以二 硫键与蛋白质相连。 (b) FeMo辅因子， 这一新的Fe-S复合 物含有1个Mo，7个 Fe和9个S，通过Mo 和Fe与蛋白质相连。
( 五) 固氮的基因表达调控 1.自身固氮
克氏杆菌的固氮基因研究的较深入，其固氮基因群除固氮 酶基因nif HDK外，nif E,nif N,nif B和nif Q与FeMoco的加工 或装配有关， nif S及nif U的产物参与FeMoco的加工和装配， nif M的产物参与FeMoco的加工成熟，nif F和nif J的产物与固 氮酶系的电子传递有关，nif LA是调控基因。
谷氨酰胺合成酶的共价修饰，12个单体可以被 逐步腺苷酸化而失活。
细菌谷氨酰胺 合成酶的结构: (a) 十二聚体 由两个六聚体 组成。 (b)谷 氨酰胺合成酶 的分子结构。
谷氨酰胺合成 酶的反馈抑制 别购调控
Allosteric regulation of glutamine synthetase. The enzyme undergoes cumulative regulation by six end products of glutamine metabolism. Alanine and glycine probably serve as indicators of the general status of amino acid metabolism in the cell.
第31章 31章 氨基酸及其重 要衍生物的生 物合成
一、概论 （一）氨的来源
（二）碳骨架的来源
（三） 生物固氮
Nitrogen fixation by the nitrogenase complex. Electrons are transferred from pyruvate to dinitrogenase via ferredoxin (or flavodoxin) and dinitrogenase reductase. Dinitrogenase reductase reduces dinitrogenase one electron at a time, with at least six electrons required to fix one molecule of N2. An additional two electrons are used to reduce 2 H+ to H2 in a process that obligatorily accompanies nitrogen fixation in anaerobes, making a total of eight electrons required per N2 molecule.
Enzymes and cofactors of the nitrogenase complex. (a) In this ribbon diagram, the dinitrogenase subunits are shown in gray and pink, the dinitrogenase reductase subunits in blue and green. The bound ADP is red. Note the 4Fe-4S complex (Fe atoms orange, S atoms yellow) and the iron-molybdenum cofactor (Mo black, homocitrate light gray). The P clusters (bridged pairs of 4Fe-4S complexes) are also shown. (b) The dinitrogenase complex cofactors without the protein (colors as in (a)). (c) The iron-molybdenum cofactor contains 1 Mo (black), 7 Fe (orange), 9 S (yellow), and one molecule of homocitrate (gray).
红色为固氮菌类菌 体，蓝色为类菌体 周膜，黄色和绿色 为被感染细胞的细 胞核。
Nitrogen-fixing nodules. (a) Root nodules of bird’s-foot trefoil, a legume. The flower of this common plant is shown in the inset. (b) Artificially colorized electron micrograph of a thin section through a pea root nodule. Symbiotic nitrogen-fixing bacteria, or bacteroids (red), live inside the nodule cells, surrounded by the peribacteroid membrane (blue). Bacteroids produce the nitrogenase complex that converts atmospheric nitrogen (N2) to ammonium (NH4+ ); without the bacteroids, the plant is unable to utilize N2. The infected root cells provide some factors essential for nitrogen fixation, including leghemoglobin; this heme protein has a very high binding affinity for oxygen, which strongly inhibits nitrogenase. (The cell nucleus is shown in yellow/green. Not visible in this micrograph are other organelles of the infected root cell that are normally found in plant cells.)
循环串联系统调 控谷氨酰胺合成 酶的共价修饰
Biosynthesis of proline and arginine from glutamate in bacteria. All five carbon atoms of proline arise from glutamate. In many organisms, glutamate dehydrogenase is unusual in that it uses either NADH or NADPH as a cofactor. The same may be true of other enzymes in these pathways. The -semialdehyde in the proline pathway undergoes a rapid, reversible cyclization to △ 1-pyrroline-5carboxylate (P5C), with the equilibrium favoring P5C formation. Cyclization is averted in the ornithine/arginine pathway by acetylation of the α-amino group of glutamate in the first step and removal of the acetyl group after the transamination. Although some bacteria lack arginase and thus the complete urea cycle, they can synthesize arginine from ornithine in steps that parallel the mammalian urea cycle, with citrulline and argininosuccinate as intermediates. Here, and in subsequent figures in this chapter, the reaction arrows indicate the linear path to the final products, without considering the reversibility of individual steps. For example, the second step of the pathway leading to arginine, catalyzed by Nacetylglutamate dehydrogenase, is chemically similar to the glyceraldehyde 3-phosphate dehydrogenase reaction in glycolysis and is readily reversible.
2.共生固氮
根瘤菌的结瘤基因nod A,B,C 是细胞分裂和宿主根毛弯曲 所必需的，核苷酸序列较保守，nod EF,nod G,nod H或nod LMN 参与宿主的选择，与根毛弯曲的位置，细胞分裂的效率和持久 性有关。 固氮基因分为nif基因和fix基因两群，前者与克氏杆菌的 固氮基因相似，后者的功能有待进一步研究。 豆科植物的早期结瘤素基因与根瘤的早期形成有关，晚期 结瘤素基因表达多种酶和一些功能不清的结瘤素。