Benzophenone-containing cholesterol surrogates: synthesis and biological evaluation

Synthetic Lipid Biology

Cells contain thousands of different lipids. Their rapid and redundant metabolism, dynamic movement, and many interactions with other biomolecules have justly earned lipids a reputation as a vexing class of molecules to understand. Further, as the cell's hydrophobic metabolites, lipids assemble into supramolecular structures─most commonly bilayers, or membranes─from which they carry out myriad biological functions. Motivated by this daunting complexity, researchers across disciplines are bringing order to the seeming chaos of biological lipids and membranes. Here, we formalize these efforts as "synthetic lipid biology". Inspired by the idea, central to synthetic biology, that our abilities to understand and build biological systems are intimately connected, we organize studies and approaches across numerous fields to create, manipulate, and analyze lipids and biomembranes. These include construction of lipids and membranes from scratch using chemical and chemoenzymatic synthesis, editing of pre-existing membranes using optogenetics and protein engineering, detection of lipid metabolism and transport using bioorthogonal chemistry, and probing of lipid-protein interactions and membrane biophysical properties. What emerges is a portrait of an incipient field where chemists, biologists, physicists, and engineers work together in proximity─like lipids themselves─to build a clearer description of the properties, behaviors, and functions of lipids and membranes.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP photophores have unique photochemical properties: upon n-π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C-H bonds; the radicals subsequently recombine, creating a stable covalent C-C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)-protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the "forbidden" (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.

Probes for studying cholesterol binding and cell biology

Cholesterol is a multifunctional lipid in eukaryotic cells. It regulates the physical state of the phospholipid bilayer, is crucially involved in the formation of membrane microdomains, affects the activity of many membrane proteins, and is the precursor for steroid hormones and bile acids. Thus, cholesterol plays a profound role in the physiology and pathophysiology of eukaryotic cells. The cholesterol molecule has achieved evolutionary perfection to fulfill its different functions in membrane organization. Here, we review basic approaches to explore the interaction of cholesterol with proteins, with a particular focus on the high diversity of fluorescent and photoreactive cholesterol probes available today.

Cholesterol-protein interaction: methods and cholesterol reporter molecules

Cholesterol is a major constituent of the plasma membrane in eukaryotic cells. It regulates the physical state of the phospholipid bilayer and is crucially involved in the formation of membrane microdomains. Cholesterol also affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Here, methods are described that are used to explore the binding and/or interaction of proteins to cholesterol. For this purpose, a variety of cholesterol probes bearing radio-, spin-, photoaffinity- or fluorescent labels are currently available. Examples of proven cholesterol binding molecules are polyene compounds, cholesterol-dependent cytolysins, enzymes accepting cholesterol as substrate, and proteins with cholesterol binding motifs. Main topics of this report are the localization of candidate membrane proteins in cholesterol-rich microdomains, the issue of specificity of cholesterol- protein interactions, and applications of the various cholesterol probes for these studies.

Protein-lipid interactions: paparazzi hunting for snap-shots

Photoactivatable groups meeting the criterion of minimal perturbance allow the investigation of interactions in biological samples. Here, we review the application of photoactivatable groups in lipids enabling the study of protein-lipid interactions in (biological) membranes. The chemistry of various photoactivatable groups is summarized and the specificity of the interactions detected is discussed. The recent introduction of 'click chemistry' in photocrosslinking of membrane proteins by photo-activatable lipids opens new possibilities for the analysis of crosslinked products and will help to close the gap between proteomics and lipidomics.

Practical synthesis of 3beta-amino-5-cholestene and related 3beta-halides involving i-steroid and retro-i-steroid rearrangements

Derivatives of 3beta-amino-5-cholestene (3beta-cholesterylamine) are of substantial interest as cellular probes and have potential medicinal applications. However, existing syntheses of 3beta-amino-5-cholestene are of limited preparative utility. We report here a practical method for the stereoselective preparation of 3beta-amino-5-cholestene, 3beta-chloro-5-cholestene, 3beta-bromo-5-cholestene, and 3beta-iodo-5-cholestene from inexpensive cholesterol. A sequential i-steroid/retro-i-steroid rearrangement promoted by boron trifluoride etherate and trimethylsilyl azide converted cholest-5-en-3beta-ol methanesulfonate to 3beta-azido-cholest-5-ene with retention of configuration in 93% yield.

Steroids linked with amide bond-extended cholesterol

New type of linear cholesterol-like molecules based on cholesterol extended by attachment of etienic acid derivatives was designed and oligosteroids with two to four units were synthesized. Amide bond was used for inter steroid connections and 1-hydroxybenzotriazole active ester method was adapted for their formations. Use of disteroids as larger building blocks was applied.

Synthesis and convenient functionalization of azide-labeled diacylglycerol analogues for modular access to biologically active lipid probes

Cell membrane lipids have been identified as key participants in cell signaling activities. One important role is their involvement as site-specific ligands in protein-membrane binding interactions, which result in the anchoring of peripheral proteins onto cellular membranes. These events generally regulate protein function and localization and have been implicated in both normal physiological processes and those pertaining to disease state onset. Thus, it is important to elucidate the details of interactions at the molecular level, such as lipid-binding specificities and affinities, the location of receptor binding domains and multivalency in binding. For this purpose, we have designed and developed azido-tagged lipid analogues as conveniently functionalizable lipid probe scaffolds. Herein, we report the design and synthesis of the initial structure of this type, diacylglycerol analogue 2, which contains an azide tag at the sn-1 position of the lipid headgroup. Direct functionalization of this compound with a range of reporter groups has been performed to illustrate the facile access to probes of use for characterizing binding. Quantitative lipid-binding studies using protein kinase C, a known DAG-binding receptor, demonstrate that these probes are active mimetics of natural DAG. Thus, these DAG probes will serve as robust sensors for studies aimed at understanding binding interactions and as precursors for the development of analogous probes of more complex phospholipids and glycolipids.

Structure and function of the sterol carrier protein-2 N-terminal presequence

Although sterol carrier protein-2 (SCP-2) is encoded as a precursor protein (proSCP-2), little is known regarding the structure and function of the 20-amino acid N-terminal presequence. As shown herein, the presequence contains significant secondary structure and alters SCP-2: (i) secondary structure (CD), (ii) tertiary structure (aqueous exposure of Trp shown by UV absorbance, fluorescence, and fluorescence quenching), (iii) ligand binding site [Trp response to ligands, peptide cross-linked by photoactivatable free cholesterol (FCBP)], (iv) selectivity for interaction with anionic phospholipid-rich membranes, (v) interaction with a peroxisomal import protein [FRET studies of Pex5p(C) binding], the N-terminal presequence increased SCP-2's affinity for Pex5p(C) by 10-fold, and (vi) intracellular targeting in living and fixed cells (confocal microscopy). Nearly 5-fold more SCP-2 than proSCP-2 colocalized with plasma membrane lipid rafts and caveolae (AF488-CTB); 2.8-fold more SCP-2 than proSCP-2 colocalized with a mitochondrial marker (Mitotracker), but nearly 2-fold less SCP-2 than proSCP-2 colocalized with peroxisomes (AF488 antibody to PMP70). These data indicate the importance of the N-terminal presequence in regulating SCP-2 structure, cholesterol localization within the ligand binding site, membrane association, and, potentially, intracellular targeting.

Cholesterol reporter molecules

Cholesterol is a major constituent of the membranes in most eukaryotic cells where it fulfills multiple functions. Cholesterol regulates the physical state of the phospholipid bilayer, affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Cholesterol plays a crucial role in the formation of membrane microdomains such as "lipid rafts" and caveolae. However, our current understanding on the membrane organization, intracellular distribution and trafficking of cholesterol is rather poor. This is mainly due to inherent difficulties to label and track this small lipid. In this review, we describe different approaches to detect cholesterol in vitro and in vivo. Cholesterol reporter molecules can be classified in two groups: cholesterol binding molecules and cholesterol analogues. The enzyme cholesterol oxidase is used for the determination of cholesterol in serum and food. Susceptibility to cholesterol oxidase can provide information about localization, transfer kinetics, or transbilayer distribution of cholesterol in membranes and cells. The polyene filipin forms a fluorescent complex with cholesterol and is commonly used to visualize the cellular distribution of free cholesterol. Perfringolysin O, a cholesterol binding cytolysin, selectively recognizes cholesterol-rich structures. Photoreactive cholesterol probes are appropriate tools to analyze or to identify cholesterol binding proteins. Among the fluorescent cholesterol analogues one can distinguish probes with intrinsic fluorescence (e.g., dehydroergosterol) from those possessing an attached fluorophore group. We summarize and critically discuss the features of the different cholesterol reporter molecules with a special focus on recent imaging approaches.

Synthesis of Benzophenone-Containing Fatty Acids

Syntheses of new benzophenone-containing fatty acids (FABPs) 1, 5, and 6 and a new route to FABP 3 are described. Combined with the known 2 and 4, these FABPs comprise a set of photoactivatable fatty acid analogues with the crosslinking site at defined distances from the carboxylic acid hydroxyl group oxygen atoms ranging from 7.9 to 25.0 A.

Cholesterol Surrogates Incorporating a Benzophenone as Part of the Sterol Tetracycle

Photoactivatable analogues 4-6 of cholesterol (1), having their cross-linking site in the ring D sterol region, have been synthesized starting from bromotetralone 14 via enantioselective Robinson annulation to enone 13 and Suzuki carbonylative coupling to the appropriate phenylboronic acid. Each of 4-6 was shown to substitute successfully for 1 in an assay of apo A-I-induced cellular cholesterol efflux, indicating that these analogues equilibrated with 1 in all major cellular pools.

Nonsteroidal Benzophenone-Containing Analogues of Cholesterol

The four benzophenones, 10-13, containing the natural side chain of cholesterol (1) have been synthesized to explore whether the tetracyclic nucleus of 1 is essential for its biochemical properties. The syntheses of analogues 10, 11, and 13 feature efficient introduction of the alkyl side chain by Suzuki coupling. Preliminary biochemical evaluation of 10 and 12 suggests that the sterol tetracyclic nucleus is not required for biological compatibility with 1.

Preparation and biochemical evaluation of fluorenone-containing lipid analogs

Syntheses are described of fatty acid analogs 5 and 6, and cholesterol (2) analogs 7 and 8 containing fluorenone groups, which are both photoactivable and fluorescent. The potential of the analogs of 2 as biochemical research tools has been demonstrated by the findings that 7 and 8 can replace 2 in apolipoprotein A-I-induced cellular efflux of 2 and that fluorescence is easily visible at the surface of smooth muscle cells equilibrated with 8.

Steroids: partial synthesis in medicinal chemistry

This article reviews the progress in the chemistry of the steroids that was published between January and December 2004. The reactions and partial synthesis of estrogens, androgens, pregnanes, cholic acid derivatives, cholestanes and vitamin D analogues are covered. There are 127 references.