Biochemistry and Evolution of Anaerobic Energy Metabolism in Eukaryotes

Summary: Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.     

Origin and Evolution of Plastids and Photosynthesis in Eukaryotes

Recent progress in understanding the origins of plastids from endosymbiotic cyanobacteria is reviewed. Establishing when during geological time the endosymbiosis occurred remains elusive, but progress has been made in defining the cyanobacterial lineage most closely related to plastids, and some mechanistic insight into the possible existence of cryptic endosymbioses perhaps involving Chlamydia-like infections of the host have also been presented. The phylogenetic affinities of the host remain obscure. The existence of a second lineage of primary plastids in euglyphid amoebae has now been confirmed, but the quasipermanent acquisition of plastids by animals has been shown to be more ephemeral than initially suspected. A new understanding of how plastids have been integrated into their hosts by transfer of photosynthate, by endosymbiotic gene transfer and repatriation of gene products back to the endosymbiont, and by regulation of endosymbiont division is presented in context.Photosynthesis is biology’s equivalent of alchemy converting a common substance (CO₂) into a precious one (reduced carbon compounds rich in chemical energy). Freely available light energy is initially converted to precious chemical energy in the form of ATP. This energy, and the reducing power generated by splitting water molecules to release electrons, is used to fix carbon from atmospheric CO₂ and generate reduced carbon compounds that underpin the biosphere. It is estimated that plants and algae convert 258 billion tons of carbon dioxide into biomass by photosynthesis annually (Geider et al. 2001). Microfossils in ancient stromatolites indicate that cyanobacterium-like organisms had invented this process—or an early, perhaps nonoxygenic, version of it—at least 3.5 byo (billions of years old) (Lowe 1980; Walter et al. 1980; Schopf 1993). These photosynthetic prokaryotes substantially predate eukaryotes, which emerged much later (Rasmussen et al. 2008; Koonin 2010). The common ancestor of all eukaryotes entered into an endosymbiotic partnership with an α-proteobacterium that evolved into the mitochondrion, now the site of aerobic respiration in most eukaryotes (Gray 2012); animals and fungi are heterotrophic descendants of this partnership. Another lineage, which eventually produced the plants, entered into a second endosymbiotic partnership, this time with a cyanobacterium, which transplanted photosynthetic alchemy into eukaryotes to create plastids (Gray and Archibald 2012). This review will highlight recent progress in our understanding of the origin and evolution of plastids.     

Evolution of DNA Replication Protein Complexes in Eukaryotes and Archaea

Background

The replication of DNA in Archaea and eukaryotes requires several ancillary complexes, including proliferating cell nuclear antigen (PCNA), replication factor C (RFC), and the minichromosome maintenance (MCM) complex. Bacterial DNA replication utilizes comparable proteins, but these are distantly related phylogenetically to their archaeal and eukaryotic counterparts at best.

Methodology/Principal Findings

While the structures of each of the complexes do not differ significantly between the archaeal and eukaryotic versions thereof, the evolutionary dynamic in the two cases does. The number of subunits in each complex is constant across all taxa. However, they vary subtly with regard to composition. In some taxa the subunits are all identical in sequence, while in others some are homologous rather than identical. In the case of eukaryotes, there is no phylogenetic variation in the makeup of each complex—all appear to derive from a common eukaryotic ancestor. This is not the case in Archaea, where the relationship between the subunits within each complex varies taxon-to-taxon. We have performed a detailed phylogenetic analysis of these relationships in order to better understand the gene duplications and divergences that gave rise to the homologous subunits in Archaea.

Conclusion/Significance

This domain level difference in evolution suggests that different forces have driven the evolution of DNA replication proteins in each of these two domains. In addition, the phylogenies of all three gene families support the distinctiveness of the proposed archaeal phylum Thaumarchaeota.     

A New Aspect to the Origin and Evolution of Eukaryotes

One of the most important omissions in recent evolutionary theory concerns how eukaryotes could emerge and evolve. According to the currently accepted views, the first eukaryotic cell possessed a nucleus, an endomembrane system, and a cytoskeleton but had an inefficient prokaryotic-like metabolism. In contrast, one of the most ancient eukaryotes, the metamonada Giardia lamblia, was found to have formerly possessed mitochondria. In sharp contrast with the traditional views, this paper suggests, based on the energetic aspect of genome organization, that the emergence of eukaryotes was promoted by the establishment of an efficient energy-converting organelle, such as the mitochondrion. Mitochondria were acquired by the endosymbiosis of ancient α-purple photosynthetic Gram-negative eubacteria that reorganized the prokaryotic metabolism of the archaebacterial-like ancestral host cells. The presence of an ATP pool in the cytoplasm provided by this cell organelle allowed a major increase in genome size. This evolutionary change, the remarkable increase both in genome size and complexity, explains the origin of the eukaryotic cell itself. The loss of cell wall and the appearance of multicellularity can also be explained by the acquisition of mitochondria. All bacteria use chemiosmotic mechanisms to harness energy; therefore the periplasm bounded by the cell wall is an essential part of prokaryotic cells. Following the establishment of mitochondria, the original plasma membrane-bound metabolism of prokaryotes, as well as the funcion of the periplasm providing a compartment for the formation of different ion gradients, has been transferred into the inner mitochondrial membrane and intermembrane space. After the loss of the essential function of periplasm, the bacterial cell wall could also be lost, which enabled the naked cells to establish direct connections among themselves. The relatively late emergence of mitochondria may be the reason why multicellularity evolved so slowly. Received: 29 May 1997 / Accepted: 9 October 1997     

Interactive Evolution Modules Promote Conceptual Change

We developed a suite of online modules () to improve student understanding of challenging concepts in our introductory biology course at the University of Wisconsin-Madison. Here we assess the effectiveness of two of the modules, Species and Speciation and Natural Selection. In Year 1, unannounced pre-tests and post-tests were used to assess students’ prior knowledge and any gains resulting from lecture attendance alone. Then to test the effectiveness of the modules we divided the class into three groups. Group 1 was assigned the interactive speciation module and their analysis of the final case study in the module was graded. Groups 2 and 3 were controls. In Group 1, the subgroup of students whose mean scores on the first two exams in the course were <80% scored an average of 10.5 percentage points better on exam 3, a significant improvement. In contrast, none of the other student groups showed significant improvements in their grades. In Year 2, we tested the effectiveness of the online modules when offered as optional, ungraded, activities. We again saw significant improvement (+3.8 percentage points) only in those students who completed the modules and whose averages on the previous two exams were <80%. The differences in improvement between years 1 and 2 suggest that it is not enough simply to provide students with tools that help them learn; they also need an incentive in the form of a grade or course credit to use the tools most effectively.     

原生动物的细胞骨架蛋白及其功能组件

目前在原生动物中发现了许多新的细胞骨架蛋白,如中心元蛋白、副鞭毛杆蛋白等。深入研究发现,原生动物的细胞骨架在细胞的模式形成,细胞核的遗传中也具有重要作用。从功能组件角度着眼研究细胞骨架的功能,将有助于了解细胞骨架的进化机制。     

On the Classification and Evolution of Protein Modules

Our efforts to classify the functional units of many proteins, the modules, are reviewed. The data from the sequencing projects for various model organisms are extremely helpful in deducing the evolution of proteins and modules. For example, a dramatic increase of modular proteins can be observed from yeast to C. elegans in accordance with new protein functions that had to be introduced in multicellular organisms. Our sequence characterization of modules relies on sensitive similarity search algorithms and the collection of multiple sequence alignments for each module. To trace the evolution of modules and to further automate the classification, we have developed a sequence and a module alerting system that checks newly arriving sequence data for the presence of already classified modules. Using these systems, we were able to identify an unexpected similarity between extracellular C1Q modules with bacterial proteins.     

Cytokinesis in Eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.     

Nonsense Mutations in Eukaryotes

Biochemistry (Moscow) - Nonsense mutations are a type of mutations which results in a premature termination codon occurrence. In general, these mutations have been considered to be among the most...     

基于生物分子网络分析的精神分裂症功能模块挖掘

精神分裂症(schizophrenia, SCZ)是一种影响人类生命健康和生存质量的复杂性精神疾病,其遗传机制涉及多个生物分子的相互作用。本研究从分子水平挖掘与SCZ相关的功能模块,可为SCZ的基础研究提供新思路。首先,通过蛋白质-蛋白质互作知识引导扩展SCZ风险基因,构建SCZ特异性基因网络;随后,利用Newman分解算法挖掘功能模块,并通过拓扑学分析及泊松分布检验确定每个功能模块的拓扑属性和核心基因;最后,对功能模块进行功能学分析,根据富集到的通路类别评估功能模块之间的相互作用。结果显示,共提取了14个功能模块,均具备无标度网络性质。对所得功能模块进行拓扑学分析,共挖掘出102个核心基因,包括已知与精神分裂症相关的基因(例如EGFR、HAX1、IL1R1、RALGDS等)和尚未有相关报道的基因(例如SVIL、DNAJA1、RABAC1、STX6等)。通过富集分析发现这些功能模块参与多条生物学通路,包括细胞凋亡、ErbB信号通路、细胞周期、磷脂酶D信号通路、PI3K-Akt信号通路等。功能模块分析显示,大部分功能模块不是单独发挥作用的,它们共同影响SCZ的发生发展,具有共享的发病机制。     

The Evolution of Functional Proteins

How did enzyme catalysts evolve? First, a single catalytic group of rudimentary effectiveness could have been incorporated into a single short peptide. In the second stage, several peptides would bind together, providing a multichain assembly with improved catalytic effectiveness. These peptides would eventually be joined together in a single chain to increase thermal stability and ensure the linked inheritance of all the elements of the complex. Finally, this rough protein would be refined and improved by classical selection processes. Evidence for the second step comes from investigation of the cDNA sequence and the genomic sequence of a gene from Tetrahymena and forms the basis of the exon microgene theory (1). This hypothesis suggests that in the early stages of the evolution of functional proteins RNA consisted of exon microgenes, each of which terminated in an amber codon (UAG) and encoded independently translated peptides. These peptides would form catalytic, multichain assemblies that were the rudimentary precursors of the enzymes we know today. In support of these ideas, complementation studies have shown that a protein "refragmented" at its exon-exon boundaries produces a functional multichain protein complex. These studies have been expanded to a more general investigation of the resilience of protein catalytic function in the face of insults to protein structural integrity.     

Molecular Evolution of Translin Superfamily Proteins Within the Genomes of Eubacteria, Archaea and Eukaryotes

Translin and its interacting partner protein, TRAX, are members of the translin superfamily. These proteins are involved in mRNA regulation and in promoting RISC activity by removing siRNA passenger strand cleavage products, and have been proposed to play roles in DNA repair and recombination. Both homomeric translin and heteromeric translin-TRAX complex bind to ssDNA and RNA; however, the heteromeric complex is a key activator in siRNA-mediated silencing in human and drosophila. The residues critical for RNase activity of the complex reside in TRAX sequence. Both translin and TRAX are well conserved in eukaryotes. In present work, a single translin superfamily protein is detected in Chloroflexi eubacteria, in the known phyla of archaea and in some unicellular eukaryotes. The prokaryotic proteins essentially share unique sequence motifs with eukaryotic TRAX, while the proteins possessing both the unique sequences and conserved indels of TRAX or translin can be identified from protists. Intriguingly, TRAX protein in all the known genomes of extant Chloroflexi share high sequence similarity and conserved indels with the archaeal protein, suggesting occurrence of TRAX at least at the time of Chloroflexi divergence as well as evolutionary relationship between Chloroflexi and archaea. The mirror phylogeny in phylogenetic tree, constructed using diverse translin and TRAX sequences, indicates gene duplication event leading to evolution of translin in unicellular eukaryotes, prior to divergence of multicellular eukayrotes. Since Chloroflexi has been debated to be near the last universal common ancestor, the present analysis indicates that TRAX may be useful to understand the tree of life.     

Functional Compartmentalization of the Translation Apparatus and Channeling of tRNA/Aminoacyl-tRNA in Cells of Higher Eukaryotes

Literature data and authors' results on the structural and functional organization of the translation apparatus in higher eukaryotes are considered. Proofs are presented of the channeling of tRNA/aminoacyl-tRNA in the course of eukaryotic protein synthesis. The concept of the shuttle role of eEF1A is grounded; the factor, being in a GTP-bound form, delivers aminoacyl-tRNA to the ribosome and then, in the having undergone to a GDP -form after hydrolysis of GTP on the ribosome, forms a complex with the deacylated tRNA and delivers it to the aminoacyl-tRNA synthetase. The notion of a translational compartment is defined.     

生物信息学方法预测蛋白质相互作用网络中的功能模块

蛋白质相互作用是大多数生命过程的基础。随着高通量实验技术和计算机预测方法的发展,在各种生物中已获得了数目十分庞大的蛋白质相互作用数据,如何从中提取出具有生物学意义的数据是一项艰巨的挑战。从蛋白质相互作用数据出发获得相互作用网络进而预测出其中的功能模块,对于蛋白质功能预测、揭示各种生化反应过程的分子机理都有着极大的帮助。我们分类概括了用生物信息学预测蛋白质相互作用功能模块的方法,以及对这些方法的评价,并介绍了蛋白质相互作用网络比较的一些方法。