Lipid Synthesis and Cellular Functions

The Major Site of Lipid Synthesis

Major site is the cellular location where lipids are synthesized. These lipids are very important in the body and include steroids and natural waxes. They are soluble in organic solvents like alcohol. The synthesis of these compounds is carried out by the smooth endoplasmic reticulum. This article will cover the synthesis of these molecules in detail. 메이저사이트

Synthesis of Lipids in Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum (sER) is a soft membrane system found in cytoplasm of most eukaryotic cells. It differs from rough endoplasmic reticulum by lacking ribosomes. sER has many roles, including the synthesis of lipids and carbohydrate molecules, protein folding, calcium concentration regulation, and drug detoxification.

Lipid synthesis takes place in a continuous network of sER compartments that are connected to each other via vesicles. Classic biochemical fractionation techniques demonstrated that most lipid synthesizing enzymes are localized to the ER. The development of new techniques, such as click-chemistry labeling, now provides more precise information on the location of sER enzymes and their interactions with other organelles.

The architecture of sER is dynamic and can change according to cell phenotype and environment. For example, hepatocytes expand the size of their sER to house cytochrome P450 enzymes that detoxify fatty acids and other metabolic waste products. In addition, lipid synthesis is regulated by differentiation programs that alter the size and morphology of cells and shift the composition of internal organelles.

Synthesis of Steroids

Several types of steroids are synthesized within the smooth endoplasmic reticulum. These are derived from cholesterol. The steroid structure consists of seventeen carbon atoms bonded in four “fused” rings: three six-membered cyclohexane rings and one five-membered cyclopentane ring. The steroid structures vary by adding different functional groups to the ring system.

The precursors of steroid hormones are synthesized in the smooth endoplasmic reticulum, and the resulting active hormones are transported to the cell surface bound to plasma proteins. Steroid hormones bind to their cell-specific receptors and initiate signaling cascades that alter cell function.

Cells that produce steroid hormones, such as the cells of the adrenal cortex and certain testicular or ovarian cells, have abundant smooth endoplasmic reticulum. Unlike rough ER, sER does not contain ribosomes and is therefore not involved in protein synthesis. However, sER is important for steroid biosynthesis and for processing exogenous substances such as drugs or toxins. These activities are carried out by a class of enzymes known as sER-associated (or soluble) protein-processing enzymes.

Synthesis of Natural Wax

Wax monoesters are a class of neutral lipids produced by most organisms that perform a wide variety of functions including the prevention of desiccation in plants and insects, sound transmission in sperm whales and regulation of buoyancy in algae. They are a significant component of mammalian sebum and meibum. They can also serve as a polishing agent and protect clothing, paints, tyres and cars. Presently they are isolated by laborious methods from natural sources such as jojoba (candelilla wax), Brazilian palm tree (carnauba wax) and lanolin. They are also chemically manufactured from petroleum in the form of paraffins, which are mixtures of normal and isoalkanes.

The mouse preputial gland is enriched in wax esters (48% of total lipid content). Analysis of membrane preparations from this organ revealed abundant wax synthase activity that efficiently converted hexadecanol and [14C]palmitoyl-CoA to the wax monoester cetylpalmitate (Fig. 3A). This activity was correlated with the presence of fatty acyl-CoA reductases and intracellular lipid transport proteins. Coexpression of the jojoba wax synthase gene with the fatty acid elongase and fatty acyl-CoA reductases in HEK 293 cells resulted in synthesis of large quantities of wax monoesters.

Synthesis of Hormones

The human body produces naturally occurring, or endogenous, steroid hormones that have diverse physiological functions. They are derived from cholesterol, a fat-soluble substance that is converted by enzymatic reactions to the pregnenolone molecule (Fig. 4-1). This molecule is further metabolized to generate the five major steroid hormones, testosterone (and its more potent metabolite, dihydrotestosterone), estradiol (estrogen), progesterone, and cortisol/corticosterone (all referred to as mineralocorticoids) and DHEA (dehydroepiandrosterone, or a glucocorticoid).

Hormones are chemical messages that are transmitted in the bloodstream to recognize specific recognition sites, or receptors, located in other cells and tissue. Then, they act to regulate the activities of these distant organs through various mechanisms.

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