MCAT Biochemistry - Lipid Structure and Function

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Lipids

characterized by insolubility in water and solubility in nonpolar organic solvents; serving vital structural, signaling, and energy storage roles

amphipathic

molecule with both hydrophilic and hydrophobic regions

Phospholipids

a phosphate and alcohol that comprise the polar head group, joined to hydrophobic fatty acid tail(s) by phosphodiester linkages; further classified according to the backbone on which the molecule is built

glycerol

three-carbon alcohol; forms phosphoglycerides or glycerophospholipids

saturated fatty acid

have only single bonds; have greater van der Waals forces and a more stable overall structure; form solids at room temperature

ex. animal fat, butter

unsaturated fatty acid

includes one or more double bonds; introduces kinks into the fatty acid chain; tend to be liquids at room temperature; make up more fluid regions of the phospholipid bilayer

ex. plant oils

Glycerophospholipids (phosphoglycerides)

contain a glycerol backbone bonded by ester linkages to two fatty acids and by a phosphodiester linkage to a highly polar head group (can be positively, negatively, or neutrally charged)

phosphatidylcholine

glycerophospholipid with a choline head group

phosphatidylethanolamine

glycerophospholipid with an ethanolamine head group

Sphingolipids

have a sphingosine or sphingoid backbone; long-chain, nonpolar fatty acid tails and polar head groups, many but not all are phospholipids

glycolipid

any lipid linked to a sugar

ceramide

the simplest sphingolipid is ceramide; has a single hydrogen atom as its head group

Sphingomyelins (sphingophospholipids)

also phospholipids; either phosphocholine or phosphoethanolamine as a head group; no net charge; major components in the plasma membranes of cells producing myelin

glycosphingolipids

Sphingolipids with head groups composed of sugars bonded by glycosidic linkages; found mainly on the outer surface of the plasma membrane

Cerebrosides

glycosphingolipids with single sugar

globosides

glycosphingolipids with two or more sugar; neutral glycolipids, no charge at normal pH

Gangliosides

glycolipids that have polar head groups composed of oligosaccharides with one or more N-acetylneuraminic acid molecules at the terminus and a negative charge; play a major role in cell interaction, recognition, and signal transduction

N-acetylneuraminic acid (NANA; sialic acid)

negatively charged at physiological pH

Waxes

esters of long-chain fatty acids with long-chain alcohols; pliable solids at room temperature; protection for both plants (surface coating to prevent excessive evaporation and to protect against parasites) and animals (prevent dehydration, as a water-repellant to keep skin and feathers dry, and as lubricant).

Terpenes

a class of lipids built from isoprene (C5H8) moieties and share a common structural pattern with carbons grouped in multiples of five: a single terpene unit contains two isoprene units; the metabolic precursors to steroids and other lipid signaling molecules; produced mainly by plants and also by some insects; generally strongly scented

ex. turpentine, a derivative of resin

Monoterpenes (C10H16)

contain two isoprene units

ex. essential oils and turpentine

Sesquiterpenes

contain three isoprene units

diterpenes

contain four isopreneex. vitamin A

Triterpenes

six isoprene units, can be converted to cholesterol and various steroids

tetraterpenes

eight isoprene units

ex. Carotenoids, like β-carotene and lutein

polyterpene

isoprene chains between 1000 and 5000 units long

ex. natural rubber

Terpenoids (isoprenoids)

derivatives of terpenes that have undergone oxygenation or rearrangement of the carbon skeleton; share similar characteristics with terpenes in terms of both biological precursor function and aromatic properties

steroids

metabolic derivatives of terpenes; four cycloalkane rings fused together: three cyclohexane and one cyclopentane; functionality is determined by the oxidation status of these rings, as well as the functional groups they carry

Steroid hormones

steroids that act as hormones, meaning that they are secreted by endocrine glands into the bloodstream and then travel on protein carriers to distant sites, where they can bind to specific high-affinity receptors and alter gene expression levels

ex. testosterone, estrogens, cortisol, aldosterone

Cholesterol

is a steroid of primary importance

major component of the phospholipid bilayer responsible for mediating membrane fluidity; amphipathic molecule containing both hydrophilic and hydrophobic components; At low temperatures, it keeps the cell membrane from solidifying; at high temperatures, it holds the membrane intact and prevents it from becoming too permeable

precursor to many important molecules, including steroid hormones, bile acids, and vitamin D

Prostaglandins

20-carbon molecules are unsaturated carboxylic acids derived from arachidonic acid and contain one five-carbon ring; paracrine or autocrine signaling molecules; to regulate the synthesis of cAMP

arachidonic acid

polyunsaturated omega−6 fatty acid; precursor in the formation of prostaglandins

vitamin

essential nutrient that cannot be adequately synthesized by the body and therefore must be consumed in the diet; divided into water-soluble and lipid-soluble

Vitamin A (carotene)

unsaturated hydrocarbon that is important in vision, growth and development, and immune function

retinal

a component of the light-sensing molecular system in the human eye; most

significant metabolite, the aldehyde form of vitamin A

Retinol

the storage, alcohol form of vitamin A, ingredient in skin-care products used to reduce wrinkles and other effects of skin aging

retinoic acid

carboxylic acid form of vitamin A; a hormone that regulates gene expression during epithelial development

Vitamin D (cholecalciferol)

can be consumed or formed in a UV light–driven reaction in the skin

calcitriol

the biologically active form of vitamin D; converted from cholecalciferol in liver and kidneys; increases calcium and phosphate uptake in the intestines, which promotes bone production

rickets

vitamin D defienciency; a condition seen in children characterized by underdeveloped, curved long bones as well as impeded growth

Vitamin E

a group of closely related lipids called tocopherols and tocotrienols; characterized by a substituted aromatic ring with a long isoprenoid side chain; hydrophobic

Tocopherols

biological antioxidants; aromatic ring reacts with free radicals, destroying them.; prevents oxidative damage, an important contributor to the development of cancer and aging

Vitamin K

actually a group of compounds, including phylloquinone (K1) and the menaquinones (K2); vital to the posttranslational modifications required to form prothrombin, an important clotting factor; aromatic ring of vitamin K undergoes a cycle of oxidation and reduction during the formation of prothrombin; also required to introduce calcium-binding sites on several calcium-dependent proteins

warfarin

anticoagulant; blocks the recycling of vitamin K, causing a deficiency or lowering of the active amount

Triacylglycerols (triglycerides)

three fatty acids bonded by ester linkages to glycerol; rare for all three fatty acids to be the same; nonpolar and hydrophobic

animals store in apipocytes; plants store as oil

adipocytes

Special cells in animals store large amounts of fat and are found primarily under the skin, around mammary glands, and in the abdominal cavity

Saponification

ester hydrolysis of triacylglycerols using a strong base, i.e. lye; basic cleavage of the fatty acid, leaving the sodium salt of the fatty acid and glycerol

lye

common name for sodium or potassium hydroxide

surfactant

lowers the surface tension at the surface of a liquid, serving as a detergent or emulsifier

colloid

the two phases appear to combine into a single phase

micelles

tiny aggregates of soap with the hydrophobic tails turned inward and the hydrophilic heads turned outward, thereby shielding the hydrophobic lipid tails and allowing for overall solvation

adipocere

natural saponification in corpses as triacylglycerols are hydrolyzed by naturally occurring bases

lecithins

complicated lipids, generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic