MCAT Biochemistry
This lesson discusses signaling lipids, which are lipids that activate or propagate cellular signaling cascades, coordinating an organism or tissue response. The building blocks of signaling lipids are terpenes, which are formed from the polymerization of 5-carbon isoprene units. Terpenoids are oxygenated derivatives of terpenes and have similar structural characteristics and functions. The triterpene squalene serves as the precursor to all naturally-occurring steroids, which share a hydrophobic, hydrocarbon backbone composed of four fused rings. Cholesterol is an amphipathic steroid that plays a role in mediating membrane fluidity and serves as the precursor to steroid hormones, vitamin D, and bile salts. Steroid hormones are secreted from endocrine glands into the bloodstream and include sex steroids and corticosteroids. Prostaglandins are 20-carbon hormones derived from arachidonic acid that act as paracrine and autocrine signaling molecules, regulating cyclic AMP and mediating inflammatory responses. Lastly, the fat-soluble vitamins A, D, E, and K are stored in adipose tissue for later use.
Lesson Outline
<ul> <li>Introduction to Signaling Lipids</li> <ul> <li>Signaling lipids activate or propagate cellular signals</li> <li>Can diffuse through the cell membrane without a transmembrane receptor</li> </ul> <li>Terpenes and Terpenoids</li> <ul> <li>Terpenes: lipids made from the polymerization of 5-carbon isoprene units</li> <li>Terpenoids: oxygenated derivatives of terpenes</li> <li>Terpene classification: based on the number of isoprene units, examples include monoterpenes (2 units), sesquiterpenes (3 units), diterpenes (4 units), triterpenes (6 units), tetraterpenes (8 units), and polyterpenes (more than 8 units).</li> <li>Squalene: a triterpene, precursor molecule to all naturally-occurring steroids</li> </ul> <li>Steroids</li> <ul> <li>Structure: 17 carbon backbone arranged into 4 fused rings</li> <li>Cholesterol: serves as a precursor molecule to steroid hormones, vitamin D, and bile salts</li> <li>Amphipathic molecule: regulates membrane fluidity</li> </ul> <li>Steroid Hormones</li> <ul> <li>Secreted by endocrine glands into the bloodstream</li> <li>Two types: sex steroids and corticosteroids</li> </ul> <li>Prostaglandins</li> <ul> <li>Derived from arachidonic acid</li> <li>Function as paracrine and autocrine signaling molecules</li> <li>Regulate cyclic AMP, mediating inflammatory responses</li> </ul> <li>Fat-Soluble Vitamins: A, D, E, and K</li> <ul> <li>Stored in adipose tissue</li> </ul> </ul>
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FAQs
Terpenes and terpenoids are both classes of organic compounds primarily composed of isoprene units. Terpenes are hydrocarbons, whereas terpenoids contain additional functional groups, such as oxygen, and may undergo further oxidation. Both terpenes and terpenoids can act as signaling lipids, which are bioactive lipids that transmit messages between cells and tissues to regulate various biological processes, like inflammation or hormone production.
Steroids function as signaling lipids by acting as hormones, modulating gene expression, and regulating cellular processes. They are synthesized from cholesterol, which is a precursor to all steroids. Cholesterol can be converted into various steroid hormones, such as glucocorticoids, mineralocorticoids, and sex hormones like estrogen and testosterone, which help regulate diverse physiological processes like metabolism, immune function, and reproduction.
Prostaglandins are a class of physiologically active lipid compounds derived from fatty acids, particularly arachidonic acid. They act as signaling lipids by modulating multiple cellular processes, including inflammation, blood clotting, and hormone regulation. Prostaglandins exert their effects by binding to specific cell surface receptors, initiating downstream intracellular signaling cascades that ultimately alter cell function in various ways, such as by activating enzymes, inhibiting protein synthesis, or changing ion channel activity.
The main types of fat-soluble vitamins are vitamins A, D, E, and K. Each plays a unique role in human health. Vitamin A is essential for vision, immune function, and cell differentiation. Vitamin D maintains calcium and phosphate homeostasis, promoting bone health. Vitamin E acts as a potent antioxidant, protecting cells from oxidative stress. Vitamin K is necessary for proper blood clotting and bone metabolism. Being fat-soluble, these vitamins can accumulate in the body and are stored in fatty tissues and the liver, making it essential to balance their intake to avoid potential toxicity.
Squalene is a triterpene and an important intermediate in the biosynthesis of sterols, including cholesterol, which further serves as a precursor for steroid hormones and some compounds with similar functions to terpenoids. It is synthesized from isoprene units through the mevalonate pathway in cells. Squalene then undergoes a series of enzymatic reactions that lead to the formation of cholesterol, which further serves as a precursor for steroid hormones. Additionally, squalene can be converted into other biologically active terpenoids, like ubiquinone (coenzyme Q), which plays a critical role in mitochondrial electron transport and cellular energy production.
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