lipid metabolism and pathways Flashcards
1
Q
what are the biological functions of lipids? 4
A
- essential components of cell membranes (phospholipids, glycolipids, cholesterol)
- intra and inter cellular signalling events (precursor or steroid hormones)
- energy generation and fuel storage (triglycerides)
- metabolism (bile acids)
2
Q
what are triglycerides? 4
A
- storage lipid
- constitute 90% of dietary lipids
- major form of metabolic energy storage in humans
- hydrophobic in nature
3
Q
describe the metabolism of triglycerides? 3
A
- depending on metabolic requirements there are 2 major metabolic pathways
- TGs broken into free fatty acids and glycerol. oxidation of fatty acids in the mitochondria to release energy in the form of ATP
- synthesis of fatty acids from acetyl-CoA (joined to a glycerol molecule for storage)
4
Q
what are the 3 stages to achieve complete oxidation of fatty acids?
A
- removal of glycerol and oxidation of long fatty acids to 2 carbon fragments in the for of acetyl-CoA. this is beta oxidation (oxidation of a carbon atom at a particular position relative to the carboxyl group
- oxidation of acetyl-CoA in the citric acid cycle
- transfer of electrons from reduced electron carries to the mitochondrial respiratory chain
5
Q
explain beta oxidation of fatty acids? 4
A
- occurs in mitochondria and peroxisomes
- 1st step- fatty acids activated to CoenzymeA (cytosol)
- 2nd step- transfer of acyl groups across the mitochondrial membrane (rate limiting step)
- 3rd step- progressive oxidation of fatty acids by removal of 3-carbon units to form acetyl-CoA, which enters the citric acid cycle
6
Q
explain fatty acid synthesis? 4
A
- occurs mainly in liver and adipocytes
- long carbon chain molecule built up from 2-carbon units derived from acetyl-CoA
- occurs in the cytosol
- but acetyl CoA is in the mitochondria!
7
Q
how do we get Acetyl-CoA out of the mitochondria? 2
A
- the tricarboxylate transporter takes citrate out of the mitochondria.
- citrate lyase then makes ATP convert to ADP and CoA convert ti Acetyl CoA and this converts citrate to OAA
8
Q
explain fatty acid biosynthesis? 3
A
- citrate–> ACetyl-CoA (carboxylation= rate limiting)–> malonyl CoA
- malonyl CoA and acetyl CoA both bind to fatty acid synthase
- a series of condensation reactions involving malonyl CoA adds further C2 units
9
Q
what are the rate limiting control steps of fatty acid oxidation and synthesis? 2
A
- beta oxidation= transfer of acyl-CoA into mitochondria
- fatty acid synthesis= formation of malonyl CoA from acetyl CoA, catalysed by acetyl CoA carboxylase (subject to control by glucagon and insulin)
10
Q
describe cholesterol? 3
A
- essential to life
- deposition in arteries is associated with heart disease and stroke
- in healthy organisms, balance is maintained between biosynthesis, utilisation and transport keeps harmful deposition to a minimum
11
Q
what are the physiological roles of cholesterol? 3
A
- important lipid component of biological membranes
- precursor of steroid hormones
- source of bile acids
12
Q
what are bile acids? 4
A
- polar derivatives of cholesterol
- aid in lipid digestion
- aid in lipid absorption
- aid in cholesterol excretion
13
Q
what is the structure of cholesterol?
A
- amphipathic lipid (hydrophobic and hydrophilic portions) the OH does this
- synthesised from acetyl-CoA and eliminated as bile acids
- storage form is cholesterol ester found in most tissues (formation is catalysed by cholesterol acyltransferases)
14
Q
what is the basic order for cholesterol synthesis? 5
A
- acetyl CoA
- HMG-CoA + HMG-CoA reductase
- mevalonate
- squalene
- cholesterol
15
Q
what is the rate defining step for cholesterol synthesis? 5
A
- HMG-CoA+ HMG-CoA reductase
- cholesterol is feedback inhibitor
- mevalonate is feedback inhibitor
- target site for statin drugs
- reductase activity is regulated by insulin/glucagon
