抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

The study of biomolecules in their native environments is a challenging task because of the vast complexity of cellular systems. Technologies developed in the last few years for the selective modification of biological species in living systems have yielded new insights into cellular processes. Key to these new techniques are bioorthogonal chemical reactions, whose components must react rapidly and selectively with each other under physiological conditions in the presence of the plethora of functionality necessary to sustain life. Herein we describe the bioorthogonal chemical reactions developed to date and how they can be used to study biomolecules.

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Bioorthogonal Chemistry: Fishing for Selectivity in a Sea of Functionality

Fatty acid synthase (FASN) catalyses the synthesis of fatty acids, and this synthetic pathway is upregulated in many tumours. How might FASN and increased lipogenesis be involved in cancer, and is FASN a valid therapeutic target? There is a renewed interest in the ultimate role of fatty acid synthase (FASN) — a key lipogenic enzyme catalysing the terminal steps in the de novo biogenesis of fatty acids — in cancer pathogenesis. Tumour-associated FASN, by conferring growth and survival advantages rather than functioning as an anabolic energy-storage pathway, appears to necessarily accompany the natural history of most human cancers. A recent identification of cross-talk between FASN and well-established cancer-controlling networks begins to delineate the oncogenic nature of FASN-driven lipogenesis. FASN, a nearly-universal druggable target in many human carcinomas and their precursor lesions, offers new therapeutic opportunities for metabolically treating and preventing cancer.

Fatty acid synthase and the lipogenic phenotype in cancer pathogenesis

Poly(3-hydroxyalkanoates) (PHAs) are a class of microbially produced polyesters that have potential applications as conventional plastics, specifically thermoplastic elastomers. A wealth of biological diversity in PHA formation exists, with at least 100 different PHA constituents and at least five different dedicated PHA biosynthetic pathways. This diversity, in combination with classical microbial physiology and modern molecular biology, has now opened up this area for genetic and metabolic engineering to develop optimal PHA-producing organisms. Commercial processes for PHA production were initially developed by W. R. Grace in the 1960s and later developed by Imperial Chemical Industries, Ltd., in the United Kingdom in the 1970s and 1980s. Since the early 1990s, Metabolix Inc. and Monsanto have been the driving forces behind the commercial exploitation of PHA polymers in the United States. The gram-negative bacterium Ralstonia eutropha, formerly known as Alcaligenes eutrophus, has generally been used as the production organism of choice, and intracellular accumulation of PHA of over 90% of the cell dry weight have been reported. The advent of molecular biological techniques and a developing environmental awareness initiated a renewed scientific interest in PHAs, and the biosynthetic machinery for PHA metabolism has been studied in great detail over the last two decades. Because the structure and monomeric composition of PHAs determine the applications for each type of polymer, a variety of polymers have been synthesized by cofeeding of various substrates or by metabolic engineering of the production organism. Classical microbiology and modern molecular bacterial physiology have been brought together to decipher the intricacies of PHA metabolism both for production purposes and for the unraveling of the natural role of PHAs. This review provides an overview of the different PHA biosynthetic systems and their genetic background, followed by a detailed summation of how this natural diversity is being used to develop commercially attractive, recombinant processes for the large-scale production of PHAs.

Metabolic Engineering of Poly(3-Hydroxyalkanoates): From DNA to Plastic

Polyketide biosynthesis: a millennium review.

A catalytic role for histidine 237 in rat mammary gland thioesterase II.

Publisher Summary This chapter focuses on the structure, mechanism of action, and regulation of the enzymes responsible for the biosynthesis of saturated fatty acids de novo in eukaryotes. Fatty acids fulfill a number of crucial roles in animals. They represent a major storage form of energy, they constitute essential structural components of membranes, they are used to modify and regulate the properties of many proteins through direct covalent linkage, and they perform important roles as signaling molecules in metabolic regulation. Cytosolic fatty acid synthase (FAS) is responsible for the bulk synthesis of fatty acids and in animals is most active in the “lipogenic tissues” such as liver, adipose, and lactating mammary glands. The exact role of the mitochondrial FAS is unclear, but it may provide the octanoyl precursor required for the de novo biosynthesis of lipoyl moieties that are utilized for the covalent modification of several mitochondrial enzymes, and/or fatty acids required for the remodeling of mitochondrial lipids. In the mitochondrial system, the enzymes exist as separate, freestanding proteins (type II) whereas in the cytosolic system, the enzymes are covalently linked in large multifunctional polypeptides (type I). The mitochondrial proteins closely resemble their prokaryotic type II counterparts, consistent with the hypothesis that mitochondria originated from free-living bacteria, but are nuclear encoded and possess N-terminal targeting sequences that direct them into the mitochondrial compartment.

Fatty acid synthesis in eukaryotes

The properties of the beta-ketoacyl reductase, dehydrase, and enoyl reductase components of the animal fatty acid synthase responsible for the reduction of the beta-ketoacyl moiety formed at each round of chain elongation have been studied by engineering and characterizing mutants defective in each of these three catalytic domains. These "beta-carbon processing" mutants leak the stalled four-carbon intermediates by direct transfer to CoA. However, enoyl reductase mutants leak beta-ketobutyryl, beta-hydroxybutyryl, and crotonyl moieties, a finding explained, at least in part, by the observation that the equilibrium and rate constant for the dehydrase reaction favor the formation of beta-hydroxy rather than enoyl moieties. In this regard, the type I animal fatty acid synthase resembles its type II counterpart in Escherichia coli in that both systems rely on the enoyl reductase to pull the beta-carbon processing reactions to completion. Kinetic and nucleotide binding measurements on fatty acid synthases mutated in either of the two nucleotide binding domains revealed that the NADPH binding sites are nonidentical, the enoyl reductase exhibiting higher affinity. Surprisingly, NADPH binding is also completely compromised by certain deletions and mutations in the central core region distant from the nucleotide binding sites. Comparable central core sequences are present in the structurally related modular polyketide synthases, except in those modules that lack all three beta-carbon processing enzymes. These findings suggest that the central core region of fatty acid and polyketide synthases plays an important role in facilitating the beta-carbon processing reactions.

Characterization of the β-Carbon Processing Reactions of the Mammalian Cytosolic Fatty Acid Synthase: Role of the Central Core†

Engineering of an Active Animal Fatty Acid Synthase Dimer with Only One Competent Subunit

The enzyme activity responsible for translocation of saturated acyl chains from the 4‘-phosphopantetheine of the acyl carrier protein to the active site cysteine of the β-ketoacyl synthase in the animal fatty acid synthase has been identified. An enzyme assay was devised that allows uncoupling of the interthiol transfer step from the condensation reaction. Experiments with various fatty acid synthase mutants indicate clearly that catalysis of the transfer of saturated acyl moieties from the 4‘-phosphopantetheine thiol to the active site cysteine thiol, Cys-161, is an inherent property of the β-ketoacyl synthase domain. Catalytic efficiency of the interthiol transferase increases from C2 to C12 and decreases with increasing chain-lengths beyond C12. Malonyl, β-hydroxybutyryl, and crotonyl thioesters are not substrates for the transferase, whereas the β-ketobutyryl moiety is a poor substrate. These features of the substrate specificity are exactly as predicted for a transferase that fulfills the proposed ro...

Stuart Smith

Papers

A catalytic role for histidine 237 in rat mammary gland thioesterase II.

Fatty acid synthesis in eukaryotes

Characterization of the β-Carbon Processing Reactions of the Mammalian Cytosolic Fatty Acid Synthase: Role of the Central Core†

Engineering of an Active Animal Fatty Acid Synthase Dimer with Only One Competent Subunit

Characterization of the interthiol acyltransferase reaction catalyzed by the beta-ketoacyl synthase domain of the animal fatty acid synthase.