The structural and functional coordination of glycolytic enzymes in muscle: evidence of a metabolon?

Leucine zipper-mediated targeting of multi-enzyme cascade reactions to inclusion bodies in Escherichia coli for enhanced production of 1-butanol

Metabolons in nature have evolved to facilitate more efficient catalysis of multistep reactions through the co-localization of functionally related enzymes to cellular organelles or membrane structures. To mimic the natural metabolon architecture, we present a novel artificial metabolon that was created by targeting multi-enzyme cascade reactions onto inclusion body (IB) in Escherichia coli. The utility of this system was examined by co-localizing four heterologous enzymes of the 1-butanol pathway onto an IB that was formed in E. coli through overexpression of the cellulose binding domain (CBD) of Cellulomonas fimi exoglucanase. To target the 1-butanol pathway enzymes to the CBD IB, we utilized a peptide-peptide interaction between leucine zipper (LZ) peptides. We genetically fused the LZ peptide to the N-termini of four heterologous genes involved in the synthetic 1-butanol pathway, whereas an antiparallel LZ peptide was fused to the CBD gene. The in vivo activity of the CBD IB-based metabolon was examined through the determination of 1-butanol synthesis using E. coli transformed with two plasmids containing the LZ-fused CBD and LZ-fused 1-butanol pathway genes, respectively. In vivo synthesis of 1-butanol using the engineered E. coli yielded 1.98g/L of 1-butanol from glucose, representing a 1.5-fold increase over that obtained from E. coli expressing the LZ-fused 1-butanol pathway genes alone. In an attempt to examine the in vitro 1-butanol productivity, we reconstituted CBD IB-based metabolon using CBD IB and individual enzymes of 1-butanol pathway. The 1-butanol productivity of in vitro reconstituted CBD IB-based metabolon using acetoacetyl-CoA as the starting material was 2.29mg/L/h, 7.9-fold higher than that obtained from metabolon-free enzymes of 1-butanol pathway. Therefore, this novel CBD-based artificial metabolon may prove useful in metabolic engineering both in vivo and in vitro for the efficient production of desired products.

Understanding Metabolic Regulation at a Systems Level: Metabolite Sensing, Mathematical Predictions, and Model Organisms

Metabolic networks are extensively regulated to facilitate tissue-specific metabolic programs and robustly maintain homeostasis in response to dietary changes. Homeostatic metabolic regulation is achieved through metabolite sensing coupled to feedback regulation of metabolic enzyme activity or expression. With a wealth of transcriptomic, proteomic, and metabolomic data available for different cell types across various conditions, we are challenged with understanding global metabolic network regulation and the resulting metabolic outputs. Stoichiometric metabolic network modeling integrated with "omics" data has addressed this challenge by generating nonintuitive, testable hypotheses about metabolic flux rewiring. Model organism studies have also yielded novel insight into metabolic networks. This review covers three topics: the feedback loops inherent in metabolic regulatory networks, metabolic network modeling, and interspecies studies utilizing Caenorhabditis elegans and various bacterial diets that have revealed novel metabolic paradigms.

Glycolytic Enzymes Localize to Synapses under Energy Stress to Support Synaptic Function

Changes in neuronal activity create local and transient changes in energy demands at synapses. Here we discover a metabolic compartment that forms in vivo near synapses to meet local energy demands and support synaptic function in Caenorhabditis elegans neurons. Under conditions of energy stress, glycolytic enzymes redistribute from a diffuse localization in the cytoplasm to a punctate localization adjacent to synapses. Glycolytic enzymes colocalize, suggesting the ad hoc formation of a glycolysis compartment, or a "glycolytic metabolon," that can maintain local levels of ATP. Local formation of the glycolytic metabolon is dependent on presynaptic scaffolding proteins, and disruption of the glycolytic metabolon blocks the synaptic vesicle cycle, impairs synaptic recovery, and affects locomotion. Our studies indicate that under energy stress conditions, energy demands in C. elegans synapses are met locally through the assembly of a glycolytic metabolon to sustain synaptic function and behavior. VIDEO ABSTRACT.

Novel myristoylation of the sperm-specific hexokinase 1 isoform regulates its atypical localization

The hexokinase 1 variant in mammalian spermatozoa (HK1S) has a unique N-terminus and this isoform atypically localizes to the plasma membrane. However, the mechanism of this process currently remains ambiguous. In this report, we show that fatty acylation underlies the specific sorting of HK1S. Employing chimeric reporter constructs, we first established that compartmentalization of HK1S does not function exclusively in sperm cells and that this feature is swappable to somatic HEK293 cells. Although the N-terminus lacks the classical consensus signature for myristoylation and the sequence-based predictions fail to predict myristoylation of HK1S, complementary experimental approaches confirmed that HK1S is myristoylated. Using live-cell confocal microscopy, we show that the mutation of a single amino acid, the myristoyl recipient Gly(2), impedes the prominent feature of plasma membrane association and relocates the enzyme to the cytosol but not the nucleus. Additionally, substitutions of the putatively palmitoylated Cys(5) is also reflected in a similar loss of compartmentalization of the protein. Taken together, our findings conclusively demonstrate that the N-terminal 'MGQICQ' motif in the unique GCS domain of HK1S acquires hydrophobicity by dual lipidic modifications, N-myristoylation and palmitoylation, to serve the requirements for membranous associations and thus its compartmentalization.

The terminal enzymes of cholesterol synthesis, DHCR24 and DHCR7, interact physically and functionally

Cholesterol is essential to human health, and its levels are tightly regulated by a balance of synthesis, uptake, and efflux. Cholesterol synthesis requires the actions of more than twenty enzymes to reach the final product, through two alternate pathways. Here we describe a physical and functional interaction between the two terminal enzymes. 24-Dehydrocholesterol reductase (DHCR24) and 7-dehydrocholesterol reductase (DHCR7) coimmunoprecipitate, and when the DHCR24 gene is knocked down by siRNA, DHCR7 activity is also ablated. Conversely, overexpression of DHCR24 enhances DHCR7 activity, but only when a functional form of DHCR24 is used. DHCR7 is important for both cholesterol and vitamin D synthesis, and we have identified a novel layer of regulation, whereby its activity is controlled by DHCR24. This suggests the existence of a cholesterol "metabolon", where enzymes from the same metabolic pathway interact with each other to provide a substrate channeling benefit. We predict that other enzymes in cholesterol synthesis may similarly interact, and this should be explored in future studies.