cellular metabolism 4

fatty acid biosynthesis

requires acetyl-CoA and CO2
acetyl-CoA is made in the mitochondria by the pyruvate dehydrogenase complex
carboxybiotin and acetyl-CoA are used to make malonyl-CoA
acetyl-CoA carboxylase is the key regulatory enzyme for this conversion and the enzyme is lo

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is phosphate a good LG?

yes

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what is the key regulatory step of fatty acid biosynthesis?

synthesis of malonyl-CoA by acetyl-CoA carboxylase

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acetyl-CoA and malonyl- CoA are transferred to ... for the pathway

ACP

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acyl carrier protein (ACP)

has the same side chain as CoA
the side chain is covalently linked to the enzyme
this means that intermediates are moved from one active site to the next as we move from one step to the next

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the biosynthetic pathway has four steps:

1/ condensation (bond formation) to form a 4 carbon chain
2/ NADPH-dependent reduction of ketone
3/ formation of double bond by dehydration
4/ NADPH-dependent reduction of double bond
- each cycle adds a saturated C2 unit
in mammals a multi-enzyme complex

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fatty acid degradation

1/ oxidative process
2/ located in mitochondria and peroxisomes
3/ CoA intermediates
4/ produces energy
5/ NAD+ and FAD are reduced
6/ separate enzymes
7/ CO2 and biotin not required

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biosynthesis

1/ reductive process
2/ located in cytosol
3/ ACP bound intermediates
4/ uses energy
5/ NADPH is oxidised
6/ polyenzymes (in mammals)
7/ CO2 and biotin required (to make malonyl-CoA)

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branched-chain lipids and steroids

lovastatin
targets pathway of cholesterol biosynthesis

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there are several types of isoprenoid lipids:

1/ acylic eg geranylgeranyl-pyrophosphate- acid derivative is linked to N- terminus of proteins to anchor them to membranes
2/ cholesterol- an important component of eukaryotic cell membranes
3/ steroid hormones made from cholesterol- eg testosterone, pro

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a number of diseases are associated with

isoprenoid lipids

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the most common is

high blood cholesterol which causes atherosclerosis and is a risk factor for heart disease, strokes etc

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the mevalonic acid pathway

HMG-CoA reductase is the drug target for statins (cholesterol-lowering agents)
mevalonate is the key intermediate in this pathway, thus you may hear is being called the 'mevalonate pathway'

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isoprenoids

IPP and DMAPP are condensed together
successive molecules of IPP are added to make longer isoprenoid lipids
dimerisation of farnasyl-pyrophosphate and reduction with NADPH produces squalene (through cont condensation rxns)

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cyclisation of squalene

step 1: epoxidation by a cytochrome P450 enzyme using NADPH and dioxygen
step 2: proton adds to epoxide ring
step 3: skeleton 'zips up' to form steroid backone
step 4: rearrangement forms lanosterol
squalene is a strained ring so will be easily opened

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steroids

many have hormone or other functions
lanosterol- all other enzymes are made from this enzyme
C21= 21 carbon atoms
there are a large number of steroids we can make and these often have biological actiivty
steroids are highly lipophilic and will pass easily

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cellular metabolism 4

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