Localization of thioesterase II, the chain-length regulatory enzyme of milk fatty acid synthesis, in rat mammary gland epithelial cells

Animal Fatty Acid Synthase: A Chemical Nanofactory

Fatty acids are crucial molecules for most living beings, very well spread and conserved across species. These molecules play a role in energy storage, cell membrane architecture, and cell signaling, the latter through their derivative metabolites. De novo synthesis of fatty acids is a complex chemical process that can be achieved either by a metabolic pathway built by a sequence of individual enzymes, such as in most bacteria, or by a single, large multi-enzyme, which incorporates all the chemical capabilities of the metabolic pathway, such as in animals and fungi, and in some bacteria. Here we focus on the multi-enzymes, specifically in the animal fatty acid synthase (FAS). We start by providing a historical overview of this vast field of research. We follow by describing the extraordinary architecture of animal FAS, a homodimeric multi-enzyme with seven different active sites per dimer, including a carrier protein that carries the intermediates from one active site to the next. We then delve into this multi-enzyme's detailed chemistry and critically discuss the current knowledge on the chemical mechanism of each of the steps necessary to synthesize a single fatty acid molecule with atomic detail. In line with this, we discuss the potential and achieved FAS applications in biotechnology, as biosynthetic machines, and compare them with their homologous polyketide synthases, which are also finding wide applications in the same field. Finally, we discuss some open questions on the architecture of FAS, such as their peculiar substrate-shuttling arm, and describe possible reasons for the emergence of large megasynthases during evolution, questions that have fascinated biochemists from long ago but are still far from answered and understood.

Combining metabolic engineering and process optimization to improve production and secretion of fatty acids

Microbial oils are sustainable alternatives to petroleum for the production of chemicals and fuels. Oleaginous yeasts are promising source of oils and Yarrowia lipolytica is the most studied and engineered one. Nonetheless the commercial production of biolipids is so far limited to high value products due to the elevated production and extraction costs. In order to contribute to overcoming these limitations we exploited the possibility of secreting lipids to the culture broth, uncoupling production and biomass formation and facilitating the extraction. We therefore considered two synthetic approaches, Strategy I where fatty acids are produced by enhancing the flux through neutral lipid formation, as typically occurs in eukaryotic systems and Strategy II where the bacterial system to produce free fatty acids is mimicked. The engineered strains, in a coupled fermentation and extraction process using alkanes, secreted the highest titer of lipids described so far, with a content of 120% of DCW.

Crystal Structure and Substrate Specificity of Human Thioesterase 2: INSIGHTS INTO THE MOLECULAR BASIS FOR THE MODULATION OF FATTY ACID SYNTHASE

The type I fatty acid synthase (FASN) is responsible for the de novo synthesis of palmitate. Chain length selection and release is performed by the C-terminal thioesterase domain (TE1). FASN expression is up-regulated in cancer, and its activity levels are controlled by gene dosage and transcriptional and post-translational mechanisms. In addition, the chain length of fatty acids produced by FASN is controlled by a type II thioesterase called TE2 (E.C. 3.1.2.14). TE2 has been implicated in breast cancer and generates a broad lipid distribution within milk. The molecular basis for the ability of the TE2 to compete with TE1 for the acyl chain attached to the acyl carrier protein (ACP) domain of FASN is unknown. Herein, we show that human TE1 efficiently hydrolyzes acyl-CoA substrate mimetics. In contrast, TE2 prefers an engineered human acyl-ACP substrate and readily releases short chain fatty acids from full-length FASN during turnover. The 2.8 Å crystal structure of TE2 reveals a novel capping domain insert within the α/β hydrolase core. This domain is reminiscent of capping domains of type II thioesterases involved in polyketide synthesis. The structure also reveals that the capping domain had collapsed onto the active site containing the Ser-101-His-237-Asp-212 catalytic triad. This observation suggests that the capping domain opens to enable the ACP domain to dock and to place the acyl chain and 4'-phosphopantetheinyl-linker arm correctly for catalysis. Thus, the ability of TE2 to prematurely release fatty acids from FASN parallels the role of editing thioesterases involved in polyketide and non-ribosomal peptide synthase synthases.

Regulation of lipid synthesis genes and milk fat production in human mammary epithelial cells during secretory activation

Expression of genes for lipid biosynthetic enzymes during initiation of lactation in humans is unknown. Our goal was to study mRNA expression of lipid metabolic enzymes in human mammary epithelial cell (MEC) in conjunction with the measurement of milk fatty acid (FA) composition during secretory activation. Gene expression from mRNA isolated from milk fat globule (MFG) and milk FA composition were measured from 6 h to 42 days postpartum in seven normal women. Over the first 96 h postpartum, daily milk fat output increased severalfold and mirrored expression of genes for all aspects of lipid metabolism and milk FA production, including lipolysis at the MEC membrane, FA uptake from blood, intracellular FA transport, de novo FA synthesis, FA and glycerol activation, FA elongation, FA desaturation, triglyceride synthesis, cholesterol synthesis, and lipid droplet formation. Expression of the gene for a key lipid synthesis regulator, sterol regulatory element-binding transcription factor 1 (SREBF1), increased 2.0-fold by 36 h and remained elevated over the study duration. Expression of genes for estrogen receptor 1, thyroid hormone-responsive protein, and insulin-induced 2 increased progressively to plateau by 96 h. In contrast, mRNA of peroxisome proliferator-activated receptor-γ decreased severalfold. With onset of lactation, increased de novo synthesis of FA was the most prominent change in milk FA composition and mirrored the expression of FA synthesis genes. In conclusion, milk lipid synthesis and secretion in humans is a complex process requiring the orchestration of a wide variety of pathways of which SREBF1 may play a primary role.

Correlation between the content of intermediate chain-length fatty acids and copper in the milk of Fulani women

Intermediate chain-length fatty acids (C10-C14) in human milk triglycerides provide an easily absorbable fuel that provide a significant amount of the energy needed for growth during the first few months of life. The C10-C14 fatty acid and trace mineral content of human milk is variable. In this report we examined the relationship between the content of calcium, copper, magnesium, manganese, phosphorus, and zinc, and the weight percentage of C10-C14 fatty acids in milk from 33 Fulani women in northern Nigeria between 2 and 24 weeks post-gestation. The milk from these women contained proportions of C10-C14 fatty acids that were comparable to those reported for other populations around the world, as were the concentrations of Ca, Cu, Mg, Mn, Zn and P. Significant correlations were observed between the milk content of Cu and the wt% of C10 (P=0. 005, r=0.475), C12 (P=0.001, r=0.539), C14 (P=0.44, r=0.352) and the total intermediate chain-length fatty acids (P=0.008, r=0.450). No correlations were observed between these fatty acids and any of the other five minerals. We speculate that the relationship between Cu and fatty acids could be related to a requirement for Cu by an enzyme required for C10-C14 fatty acid biosynthesis (e.g. decanoyl deacylase) in mammary tissue, or to some unique Cu binding properties of the intermediate chain length fatty acids.

Molecular cloning and sequence analysis of complementary DNA encoding rat mammary gland medium-chain S-acyl fatty acid synthetase thio ester hydrolase

Poly(A)+ RNA from pregnant rat mammary glands was size-fractionated by sucrose gradient centrifugation, and fractions enriched in medium-chain S-acyl fatty acid synthetase thio ester hydrolase (MCH) were identified by in vitro translation and immunoprecipitation. A cDNA library was constructed, in pBR322, from enriched poly(A)+ RNA and screened with two oligonucleotide probes deduced from rat MCH amino acid sequence data. Cross-hybridizing clones were isolated and found to contain cDNA inserts ranging from approximately 1100 to 1550 base pairs (bp). A 1550-bp cDNA insert, from clone 43H09, was confirmed to encode MCH by hybrid-select translation/immunoprecipitation studies and by comparison of the amino acid sequence deduced from the DNA sequence of the clone to the amino acid sequence of the MCH peptides. Northern blot analysis revealed the size of the MCH mRNA to be 1500 nucleotides, and it is therefore concluded that the 1550-bp insert (including G X C tails) of clone 43H09 represents a full- or near-full-length copy of the MCH gene. The rat MCH sequence is the first reported sequence of a thioesterase from a mammalian source, but comparison of the deduced amino acid sequences of MCH and the recently published mallard duck medium-chain S-acyl fatty acid synthetase thioesterase reveals significant homology. In particular, a seven amino acid sequence containing the proposed active serine of the duck thioesterase is found to be perfectly conserved in rat MCH.

Biosynthesis of medium-chain fatty acids by mammary epithelial cells from virgin rats

Epithelial cells were isolated from the undifferentiated mammary glands of mature virgin female rats, and their lipogenic characteristics were studied. These cells synthesized predominantly medium-chain fatty acids, albeit at a low rate. In contrast, whole tissue from mammary glands of virgin rats synthesized predominantly long-chain fatty acids at a relatively higher rate, indicating that the lipogenic activity is dominated by the adipocyte component of the gland. Enzyme assays revealed that thioesterase II, the enzyme which regulates production of medium-chain fatty acids by the fatty acid synthetase, was present at a high activity in the undifferentiated mammary epithelial cells of virgin rats. Immunohistochemical studies confirmed this observation and showed that the regulatory enzyme was present exclusively in the epithelial cells lining the alveolar and ductal elements of the undifferentiated gland. This study demonstrates that the potential to elaborate tissue-specific medium-chain fatty acids is already expressed in the undifferentiated tissue of virgin rats and is not acquired as a result of the differentiation associated with the lactogenic phase of development. In this species mammary epithelial cells apparently synthesize predominantly medium-chain fatty acids at all stages of development, and only the overall rate of synthesis is increased on induction of the fatty acid synthetase during lactogenesis.