With what starting material are all lipids are built? Describe the process that a long carbon chain is rearranged into a multi-cyclic molecule that is basis for steroids?
Please explain in detail
http://lipidlibrary.aocs.org/Lipids/tag2/index.htm
Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks": ketoacyl and isoprene groups
b. Biosyethesis: The building block from which all carbon atoms of steroids are derived is the two carbon acetyl group of acetyl-CoA.
Stage 1: synthesis of isopentenyl pyrophosphate from three molecules of acetyl-CoA.
Stage 2: synthesis of cholesterol.
Stage 3: conversion of cholesterol to other steroids.
All lipids are built using a starting material called acetyl-CoA. Acetyl-CoA is a small molecule that plays a central role in metabolism. It is produced from various sources like carbohydrates, fats, and proteins through several enzymatic reactions in the body.
To understand the process of how a long carbon chain is rearranged into a multi-cyclic molecule that forms the basis for steroids, we need to look at the biosynthesis of cholesterol, a well-known steroid. Cholesterol is considered a fundamental precursor for the synthesis of other steroids in the body.
The biosynthesis of cholesterol involves a series of enzymatic reactions taking place in the cytoplasm and endoplasmic reticulum of cells. Here is a step-by-step explanation of the process:
1. Condensation of Acetyl-CoA: Two molecules of acetyl-CoA are condensed to form acetoacetyl-CoA. This reaction is catalyzed by the enzyme thiolase.
2. Formation of 3-Hydroxy-3-Methylglutaryl CoA (HMG-CoA): Acetoacetyl-CoA combines with acetyl-CoA, leading to the formation of HMG-CoA. This reaction is catalyzed by the enzyme HMG-CoA synthase.
3. Formation of Mevalonate: HMG-CoA is then converted to mevalonate through a series of enzymatic steps. This process involves the intermediate formation of mevaldyl-CoA and subsequent decarboxylation. The key enzyme involved in this step is HMG-CoA reductase.
4. Conversion of Mevalonate to Isopentenyl Pyrophosphate (IPP): Mevalonate undergoes phosphorylation and decarboxylation reactions to form isopentenyl pyrophosphate (IPP). This process involves the action of the enzymes mevalonate kinase, phosphomevalonate kinase, and mevalonate diphosphate decarboxylase.
5. Formation of Squalene: Several molecules of IPP combine to form farnesylpyrophosphate (FPP). Further, two molecules of FPP join together to generate squalene. These reactions are catalyzed by the enzyme squalene synthase.
6. Cyclization of Squalene to Lanosterol: Squalene undergoes a series of complex reactions, including cyclization, hydride shifting, and ring closure, resulting in the formation of lanosterol. This process is catalyzed by a multifunctional enzyme called squalene epoxidase.
7. Conversion of Lanosterol to Cholesterol: Lanosterol is then enzymatically modified through various steps, including methylation, hydroxylation, reduction, and rearrangement, to ultimately yield cholesterol. This process involves the action of several enzymes, including cytochrome P450 enzymes.
It is important to note that the biosynthesis of steroids involves additional modifications and reactions specific to the particular steroid being synthesized. The process described above is a simplified overview of how a long carbon chain is rearranged into a multi-cyclic molecule forming the basis for steroids, specifically cholesterol.