Biochem Lecture 26- Fatty Acid Metabolism Part III

Where does in the cell does fatty acid breakdown happen?

Mitochondria (also peroxisomes and ER for VLC fatty acids).

Where in the cell does fatty acid synthesis happen and what types of cell do it? What does it need?

The cytosol, liver and adipose tissue. Acetyl CoA.

Where in the cell is acetyl CoA synthesized?

Mitochondria.

How does acetyl CoA needed for fatty acid synthesis get from the mitochondria to the cytosol?

It is transported via the citrate shuttle, where acetyl CoA and oxaloacetate are converted to citrate by citrate synthase, exported to the cytosol, and then converted back to acetyl CoA and oxaloacetate by citrate lysase. Oxaloacetate can then be recycled back to the mitochondria through producing malate (using up NADH and making NADPH) then pyruvate which can be pumped into the mitochondria and becomes oxaloacetate.

How do fatty acids reach peripheral tissues?

They are packaged into lipoproteins and transported via the circulatory system.

What are the starting materials of fatty acid synthesis?

Acetyl-CoA and NADPH.

What is the first step in synthesizing acetyl-CoA to fatty acids? What does the product do?

Conversion of acetyl-CoA (2C) to malonyl-CoA (3C) (adds a carboxyl group to acyl group) by acetyl-CoA carboxylase requiring ATP, Biotin, and CO2. Malonyl-CoA inhibits CPT-1 (to prevent beta oxidation).

What is the rate limiting enzyme for fatty acid synthesis? How is it regulated allosterically?

Acetyl-CoA carboxylase. It is activated by citrate through a feed forward mechanism (present during well fed state and indicates high acetyl-CoA). Palmitoyl/palmitic acid (end product) exerts a negative feedback inhibition on it. Other long chain fatty acids also inhibit it.

What is the second step of fatty acid synthesis (What do the products do?)?

Acetyl-CoA and malonyl-CoA both get binded to an acyl carrier protein (ACP) (by malonyl/acyl transferase) and release CoA-SH to form acetyl-ACP (primer) and malonyl-ACP (chain extender).

How is the fatty acid chain extended?

The fatty acid chain is extended by the addition of malonyl-ACP to acetyl-ACP through a series of condensation, reduction, dehydration, and another reduction reactions, ultimately forming longer fatty acid chains.

What is the condensation step of fatty acid chain synthesis?

Acetyl-ACP and malonyl-ACP are combined (using B-ketoacyl-ACP synthase) to form a acetoacetyl-ACP (4C), releasing CO2 and ACP-SH.

What is the first reduction step of fatty acid chain synthesis?

Acetoacetyl-ACP is reduced to 3-hydroxybutyryl-ACP using NADPH and H+ (by B-ketoacyl-ACP reductase) (produces NADP+).

What is the dehydration step of fatty acid chain synthesis?

3-hyrdoxybutyryl-ACP loses H2O (by B-hydroxyacyl-ACP dehydrogenase) to form crotonyl-ACP.

What is the second reduction step of fatty acid chain synthesis?

Crotonyl-ACP is reduced to butyryl-ACP using NADPH and H+ (by enoyl-ACP reductase) (produces NADP+).

How does butyryl-ACP become a fatty acid chain?

Butyryl-ACP repeats extensions using 6 malonyl-ACP to forming palmitoyl-ACP, then thioesterase replaces the ACP with CoA forming palmitate (16C). (It stops adding after 16 C)

How many NADPH are used per carbon to extend a fatty acid chain?

1. (Each cycle used 2 and extends the chain by 2 carbons)

What is fatty acid synthase?

It is a homodimer multi-enzyme complex in fatty acid synthesis. Consists of many enzymed needed for fatty acid synthesis. ACP is in the center.

How are unsaturated fatty acids synthesised?

Desturases in the ER can add double bonds to palmitic acid (16C) at the 9th carbon to form oleic acid (18C) (non essential bc we can make it). Humans cannot make other unsaturated fatty acids (essential bc needed from diet).

Why are unsaturated fatty acids needed?

Component of phosoholipid membrane.

What is the differece between coenzyme A and acyl carrier protein.

Coenzyme A is used in B oxidation, has a ADP (ribose, recognition site), a phosphopantothenic acid, and B-mercapto-ethylamine. ACP is used in fatty acid synthesis, has a long arm with a serine residue with an OH group that interacts with the phosphopantheine group.

How is NADPH supplied for fatty acid synthesis?

HMP shunt in cytosol, malate to pyruvate conversion by malic enzyme in cytosol, isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate and produces NADPH in the cytosol.

How is fatty acid synthesis hormonally regulated?

If the insulin to glucagon ratio is high it will trigger citrate lyase activation and phosphatase will activate acetyl-CoA carboxylase by triggering dephosphorylation. If the glucagon to insulin ratio is high or epinepherine is high protein kinase will trigger phosphorylation of acetyl-CoA carboxylase leading to inactivation.

What are differences between fatty acid degradation and synthesis (location, carrier, enzymes, oxidants/reductants, isomeric form)?

Degradation- mitochondria, CoA, different enzymes, NAD+, hydroxy acyl intermediate is in L-form. Synthesis- cytosol, ACP, fatty acid synthase, NADPH, hydroxy acyl intermediate is in D-form.

What is cholesterol? How is it circulated in blood?

27C, hydrocarbon tail, sterol nucleus, hydroxyl group. It is carried by lipoproteins after becoming esterified, this must happen because it is a toxic detergent when not in in the membrane (exogenous-chylomicrons, endogenous-VLDL).

What are the sources of cholesterol (%)?

75% from de novo synthesis, 25% from diet.

What are the functions of cholesterol?

Structural component in cell membranes, building block for steroid hormones, precursor to bile acids, synthesis of prenylated proteins, haem A, CoQ, and dolichol, important for production of vitamin D.

Where is cholesterol synthesized?

In the cytosol and ER (different steps in different places).

How is cholesterol synthesized?

2 acetyl CoA are converted to acetoacetyl-CoA by thiolase then reacts with another acetyl-CoA by HMG-CoA synthase to make HMG-CoA. HMG-CoA is made into mevalonate by HMG-CoA reductase, mevalonate is made into many things including squalene which becomes cholesterol.

What is the rate-limiting enzyme in cholesterol synthesis? How is it regulated?

HMG-CoA reductase. Feedback inhibition (ex: too much mevalonate), feedback control of gene expression, rate of enzyme synthesis/degradation, hormonal regulation.

How do statins inhibit cholesterol synthesis?

They are structural analogs of HMG-CoA reductase causing competitive inhibition (statins have a much higher affinity to the active site).