Signalling molecules steroids

Steroid hormones are crucial signal molecules in mammals. (The details of their physiological effects are described in Chapter 34.) Their biosynthesis begins with the desmolase reaction, which converts cholesterol to preg-... 

Cellular functions are controlled by extracellular signals such as hormones, neurotransmitters, odorants, light and other chemical or physical stimuli. Only a few of these signal molecules, e.g., the highly lipid-soluble steroids or thyroid hormones, can diffiise across the... 

In terms of their chemical structures, signalling molecules fall into five main categories (i) peptides, (ii) steroids, (iii) amino acids and their derivatives, (iv) fatty acid derivatives, and (v) nucleotides. 

Certain proteins, peptides, steroids, and other small organic molecules, serve as cell messengers or mediators of signals. Hydrophilic mediators activate receptors at the surface of the target cell. Acetylcholine is a common example of such signaling molecules. It becomes bound on the exterior surface at the acetylcholine receptor of the nervous system, opening the channel. 

Reproductive development and function depend on endocrine communication throughout the mammalian life cycle. Protein and steroid hormones, growth factors and other signalling molecules affect gene expression and protein synthesis in target cells of different tissues. In particular, fetal development, development of the reproductive tract,... 

The kinds of compounds that elicit signals in plants are similar to certain signaling molecules in mammals (Fig. 12-28). Instead of prostaglandins, plants have jas-monate instead of steroid hormones, brassinosteroids. 

Lipids represent a diverse group of compounds, which are readily soluble in organic solvents such as chloroform or toluene and are essential for the structure and function of ah hving ceUs. Lipids include oils, fatty acids, waxes, steroids (e.g., cholesterol and steroid hormones), and other related compounds. Specihc classes of lipids are the primary structural components of membranes, provide sources of metabolic fuel, and function as bioactive signaling molecules. 

This chapter examines the biosynthesis of three important components of biological membranes—phospholipids, sphingolipids, and cholesterol (Chapter 12). Triacylglycerols also are considered here because the pathway for their synthesis overlaps that of phospholipids. Cholesterol is of interest both as a membrane component and as a precursor of many signal molecules, including the steroid hormones progesterone, testosterone, estrogen, and cortisol. The biosynthesis of cholesterol exemplifies a fundamental mechanism for the assembly of extended carbon skeletons from five-carbon units. 

Figure 6.13 A framework of a covalently bound CjeHssSH monolayer (green blocks) is interrupted by thinner, also covalently bound template molecules (black holes), e.g. a quinone- or steroid-thiol. These template molecules serve as gates for electron transfer reactions and can be closed by signal molecules (grey blocks) which bind to those templates, e.g. carbohydrates which react with the C-17 substituent of steroid 11. The current of redox-active ions from the bulk water volume stops.

FXR is a well-characterized member of the metabolic subfamily of NRs and is a transcriptional sensor for bile acids [35]. Its ligands, including chenodeoxy-cholic acid (CDCA 37) (Fig. 8-10), act as signaling molecules and participate in an intricate network of interactions that ultimately govern lipid, steroid, and cholesterol homeostasis and are involved in processes such as glucose utilization, inflammation, and carcinogenesis [35]. Maloney et al. reported GW4064... 

In addition to Its structural role In membranes, discussed In Chapter 5, cholesterol Is the precursor for several Important bloactive molecules. They Include bile acids (see Figure 18-6), which are made In the liver and help emulsify dietary fats for digestion and absorption In the Intestines, steroid hormones produced by endocrine cells (e.g., adrenal gland, ovary, testes), and vitamin D produced In the skin and kidneys. Arthropods need cholesterol or other sterols to produce membranes and ecdysterold hormones, which control development however, they cannot make the precursor sterols themselves and must obtain these compounds In their diet. Another critical function of cholesterol Is Its covalent addition to Hedgehog protein, a key signaling molecule In embryonic development (Chapter 15). 

In most cases, steroid hormones (estradiol, testosterone) and other hydrophobic small signaling molecules act via endocrine signaling cascade. They are synthesized and secreted into the circulation by specialized endocrine cells, and they bind to their corresponding intracellular receptors following direct diffusion across the... 

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