Biosynthesis of Heme and Chlorophyll Precursors

四吡咯色素是一类生物分子，包括血红素、叶绿素、胆磷脂和维生素B12，这些色素在呼吸和光合作用过程中起着重要的作用，也在其他重要的酶反应中作为辅助因子存在。所有四吡咯都是由一个分支生物合成途径产生的，其中(-氨基乙酰丙酸（ALA）是其第一个普遍承诺的中间体。该项目的长期目标是确定植物和藻类中催化ALA形成和代谢的酶的结构、作用机制和调控，并了解ALA形成如何控制这些生物的光合色素含量。最近的进展是利用莱茵衣藻独特的遗传和生化潜力来实现这些目标。从衣藻中克隆到了3个衣藻基因gtr、gsa和alad，它们分别编码催化四吡咯生物合成前3步的酶（谷氨酰trna还原酶、谷氨酸1-半醛转氨酶和ALA脱水酶）。计划表达催化ALA形成关键初始步骤的克隆gtr基因，以获得足够数量的其产物，用于表征其物理和动力学特性、辅因子的含量和需求、反应机制、与其他酶的可能相互作用以及变构效应物对活性的调节。了解ALA的生物合成的重要性在于，它是细胞代谢物向四吡啶最终产物转移的初始通用代谢物，因此可能通过调节ALA的形成对细胞色素含量进行代谢控制。此外，所有植物和大多数细菌的ALA生物合成与动物不同，这表明有可能开发出专门针对ALA生物合成途径步骤的除草剂和抗生素，并且对动物无毒。在这一领域的合理进展将需要对五碳ALA生物合成途径的全面了解。虽然除草剂和抗生素的发展不是拟议研究的目标，但它们有助于说明这项研究对人类福利的潜在长期意义的各个方面。

BealeTetrapyrrole pigments comprise a family of biomolecules including hemes, chlorophylls, bilins, and vitamin B12- These pigments function as essential components in the processes of respiration and photosynthesis, and also exist as cofactors in other important enzyme reactions. All tetrapyrroles arise from a branched biosynthetic pathway that has (-aminolevulinic acid (ALA) as its first universal committed intermediate. The long-term goals of this project are to determine the structures, mechanisms of action, and regulation of the enzymes that catalyze ALA formation and metabolism in plants and algae, and to understand how ALA formation controls the photosynthetic pigment content of these organisms. Recent progress has exploited the unique genetic and biochemical potentials of the unicellular alga Chlamydomonas reinhardtii toward achieving these goals. Three Chlamydomonas genes, gtr, gsa, and alad, that encode enzymes which catalyze the first three steps of tetrapyrrole biosynthesis (glutamyl-tRNA reductase, glutamate 1-semialdehyde aminotransferase, and ALA dehydratase, respectively) have been cloned from Chlamydomonas. It is planned to express the cloned gtr gene which catalyzes the key, initial step of ALA formation, to obtain sufficient quantities of its product to be used for characterization of its physical and kinetic properties, cofactor content and requirements, reaction mechanism, possible interactions with other enzymes, and regulation of activity by allosteric effectors.The importance of understanding the biosynthesis of ALA lies in its position as the initial universal metabolite in the diversion of cellular metabolites toward tetrapyrrole end products, and therefore the likely exercise of metabolic control over cellular pigment content by regulation of ALA formation. Moreover, the fact that ALA biosynthesis in all plants and most bacteria differs from that in animals suggests that it may be possible to develop herbicides and antibiotics that specifically target steps in the ALA biosynthetic pathway, and which are nontoxic to animals. Rational progress in this area will require a thorough knowledge of the five-carbon ALA biosynthetic pathway. Although herbicide and antibiotic development are not objectives of the proposed research, they serve to illustrate aspects of the potential longer-term significance of this research for human welfare.