Steroids and Terpenoids


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Isoprene Rule

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Steroids and Terpenes

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Examples of C10 and higher terpenes, representing the four most common classes are shown in the following diagrams. Most terpenes may be structurally dissected into isopentane segments. How this is done can be seen in the diagram directly below.

From terpenes to steroids part 1: Terpenes - Endocrine system physiology - NCLEX-RN - Khan Academy

The isopentane units in most of these terpenes are easy to discern, and are defined by the shaded areas. In the case of the monoterpene camphor, the units overlap to such a degree it is easier to distinguish them by coloring the carbon chains. This is also done for alpha-pinene. In the case of the triterpene lanosterol we see an interesting deviation from the isoprene rule. This thirty carbon compound is clearly a terpene, and four of the six isopentane units can be identified. However, the ten carbons in center of the molecule cannot be dissected in this manner.


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  7. Evidence exists that the two methyl groups circled in magenta and light blue have moved from their original isoprenoid locations marked by small circles of the same color to their present location. This rearrangement is described in the biosynthesis section. Similar alkyl group rearrangements account for other terpenes that do not strictly follow the isoprene rule.

    KEGG PATHWAY: Biosynthesis of terpenoids and steroids - Reference pathway

    Polymeric isoprenoid hydrocarbons have also been identified. Rubber is undoubtedly the best known and most widely used compound of this kind. It occurs as a colloidal suspension called latex in a number of plants, ranging from the dandelion to the rubber tree Hevea brasiliensis. Bromine, hydrogen chloride and hydrogen all add with a stoichiometry of one molar equivalent per isoprene unit. Pyrolysis of rubber produces the diene isoprene along with other products.

    The double bonds in rubber all have a Z-configuration, which causes this macromolecule to adopt a kinked or coiled conformation.

    This is reflected in the physical properties of rubber. Despite its high molecular weight about one million , crude latex rubber is a soft, sticky, elastic substance. Chemical modification of this material is normal for commercial applications. Gutta-percha structure above is a naturally occurring E-isomer of rubber. Here the hydrocarbon chains adopt a uniform zig-zag or rod like conformation, which produces a more rigid and tough substance.

    Uses of gutta-percha include electrical insulation and the covering of golf balls.

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    From terpenes to steroids part 1: Terpenes

    While we can identify isoprene units within a terpenoid structure and use that in its classification, the building block for terpenoid synthesis in nature is isopentenyl diphosphate formerly called isopentenyl pyrophosphate and abbreviated IPP. There are two major routes to the synthesis of IPP; namely 1 the mevalonate pathway and 2 the 1-deoxyxylulose pathway.

    An initial trans-thioesterase process transfers the acetyl group of the first acetyl CoA to an enzymatic cysteine Reaction 1. In the Claisen condensation phase of the reaction, the alpha-carbon of a second acetyl CoA is deprotonated, forming an enolate Reaction 2. The enolate carbon attacks the electrophilic thioester carbon, forming a tetrahedral intermediate Reaction 3 which quickly collapses to expel the cysteine thiol Reaction 4 and produce acetoacetyl CoA. Acetyl CoA then reacts with the acetoacetyl CoA in an aldol-like addition. Step 3 - Reduction of the Thioester. The thioester is reduced first to an aldehyde, then to a primary alcohol by two equivalents of NADPH producing R -mevalonate.

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    The enzyme catalyzing this reaction is the target of the statin family of cholesterol-lowering drugs. Finally isopentenyl diphosphate IPP , the 'building block' for all isoprenoid compounds, is formed from a decarboxylation-elimination reaction. In the next step of isoprenoid biosynthesis, the two five-carbon isomers condense to form a carbon isoprenoid product called geranyl diphosphate GPP. The first step is ionization of the electrophile - in other words, the leaving group departs and a carbocation intermediate is formed.

    In this case, the pyrophosphate group on DMAPP is the leaving group, and the electrophilic species is the resulting allylic carbocation. In the condensation addition step, the C 3 -C 4 double bond in IPP attacks the positively-charged C 1 of DMAPP, resulting in a new carbon-carbon bond and a second carbocation intermediate, this time at a tertiary carbon.

    In the elimination phase, proton abstraction leads to re-establishment of a double bond in the GPP product. Notice that the enzyme specifically takes the pro-R proton in this step. To continue the chain elongation process, another IPP molecule can then condense, in a very similar reaction, with C 1 of geranyl diphosphate to form a carbon product called farnesyl diphosphate FPP. How do we know that these are indeed S N 1-like mechanisms with carbocation intermediates, rather than concerted S N 2-like mechanisms?

    First of all, recall that the question of whether a substitution is dissociative S N 1-like or associative S N 2-like is not always clear-cut - it could be somewhere in between, like the protein prenyltransferase reaction. The protein prenyltransferase reaction and the isoprenoid chain elongation reactions are very similar: the electrophile is the same, but in the former the nucleophile is a thiolate, while in the latter the nucleophile is a pi bond.

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