Biochem - fatty acid metabolism

What are the functions of fatty acids?

• Energy storage and production

• Structural components of membranes

• Hormones derived from fatty acids

How are fatty acids present within cells?

• Conjugated with glycerol to form lipids

• Triacylglycerol (TAG) consists of 3 fatty acids linked by ester bonds to glycerol

• Lipids are highly hydrophobic and so are not easily transported

How can TAG be used for energy within the cell?

• All tissues (except the brain) can oxidise fatty acids derived from TAG to produce energy

• TAG is not used in most cases until glucose supplies are low

• TAG breakdown in adipose tissues is broken down by triacylglycerol lipase

• TAG + 3H2O —> glycerol + 3 fatty acids

• Lipase is hormonally activated by glucagon

• Fatty acids are transported in the blood complexed with serum albumin

How are fatty acids oxidised within cells?

• Three step process

• Free fatty acids in the cytosol are activated forming fatty acyl-CoA

• Fatty acyl-CoA uptake into mitochondria

• B-oxidation pathway in mitochondrial matrix

What happens in the first step of fatty acid oxidation? (activation)

• Add coenzyme A via a thioester (S on the CoA)

• ATP and fatty acid are reacted to form fatty acyl-AMP as an intermediate

• Activation costs the equivalent of 2 ATP molecules

• AMP + ATP —> 2ADP

What happens during the second step of fatty acid oxidation? (uptake by mitochondria)

• Inner mitochondrial membrane is not permeable to fatty acids or fatty acyl CoA

• Carnitine shuttle is used to transport fatty acyl CoA from the cytoplasm to the mitochondrial matrix

• Fatty acyl CoA cannot pass but fatty acyl-carnitine can - react together

• Carrier protein exchanges fatty acyl-carnitine with a carnitine molecule

• Reforms fatty acyl-CoA when inside the mitochondrial matrix

What happens during the third step of fatty acid oxidation? (oxidation in the mitochondria)

• Called B-oxidation as the B-carbon (C3) is oxidised

• H is removed from a and B carbons by acyl CoA dehydrogenase - contains coenzyme FAD

• Forms a double bond between the two carbons - electrons are delivered to coenzyme Q in the ETC

• Hydration of the double bond to form hydroxyacyl-CoA

• C-OH is oxidised to C=O, forming NADH - oxidised in the ETC to produce ATP

• CoASH attacks the B carbon to produce acetyl CoA and a fatty acyl CoA that is 2 carbons shorter than the original molecule

• Acetyl CoA is oxidised by the citric acid cycle

• Process repeats for even chain length fatty acids until all the molecule is converted into acetyl CoA

• Odd chain length fatty acids yield acetyl CoA and propionyl CoA - mammals can make glucose from propionyl CoA

What is the glyoxylate cycle?

• Allows plants, fungi and bacteria to make sugars from fatty acids

• Animals cannot convert acetyl CoA (produced by fatty acid oxidation) to sugars but plants, fungi and bacteria can

• Plants, fungi and bacteria use the glyoxylate cycle

How does the glyoxylate cycle occur?

• 2 acetyl CoA —> malate —> oxaloacetate

• Bypasses a-ketoglutarate and succinyl CoA - two reactions that release CO2

• Can either proceed via glyoxylate or succinate from isocitrate - both produce malate

• Produces 2x malate where the citric acid cycle only produces 1

• Can keep one malate in the cycle and use the other to form oxaloacetate and undergo gluconeogenesis

What are the reactions in the glyoxylate cycle?

• Acetyl CoA + oxaloacetate —> citrate

• Citrate —> isocitrate

• Isocitrate —> glyoxylate (catalysed by isocitrate lyase) + succinate

• Succinate —> fumarate (+ FADH2)

• Fumarate —> malate

• Glyoxylate + acetyl CoA —> malate (catalysed by malate synthase)

• Malate —> oxaloacetate (+ NADH)

How are fatty acids synthesised within cells?

• Fatty acid biosynthesis has the same intermediates as B oxidation but occurs in the cytoplasm

• Substrates are acetyl CoA and NADPH

• Citrate shuttle ‘delivers’ acetyl CoA from the mitochondria to the cytoplasm

• Sources of NADPH for fatty acid synthesis are the pentose phosphate pathway and the citrate shuttle

What is the mechanism of fatty acid biosynthesis?

• Malonyl CoA is synthesised from acetyl CoA, CO2 and ATP (catalysed by acetyl CoA carboxylase - biotin coenzyme)

• Acetyl group is added using malonyl CoA as a donor

• C=O group on B carbon is reduced to C-OH by NADPH

• Dehydration generates a double bond between a and B carbons (2 + 3)

• Reduction of the double bond to a single bond

• Fatty acyl-ACP can re-enter the process or process is stopped by removing ACP (catalysed by fatty acyl-ACP thioesterase)

What is special about enzymes involved in fatty acid synthesis?

• Carried out by a single multifunctional enzyme - exists as a dimer

• Arrangement eases movement of the fatty acyl chain through the complex process while anchored to ACP - doesn’t release and rebind intermediates

What is the structure of fatty acid synthase?

• 3 domains with an interdomain region that allows dimerization

• N-terminal

• Domain I: KS, MAT, DH

• KS - B-ketoacyl-ACP synthase; MAT - malonyl/acetyl-CoA-ACP transacylase; DH - B-hydroxyacyl ACP dehydratase

• Interdomain region

• Domain II: ER, KR, ACP

• ER - enoyl-ACP reductase; KR - B-ketoacyl-ACP reductase

• Domain III: TE

• TE - thioesterase

• C-terminal

Which two features of fatty acid synthesis and oxidation can be regulated by the cell?

• The enzymes

• The processes

How are enzymes regulated in fatty acid synthesis and oxidation?

• Acetyl CoA carboxylase is inhibited by phosphorylation

• AMP-activated protein kinase is a major kinase

• Others are controlled by glucagon and adrenaline

• Insulin signalling favours fatty acid synthesis by activating a phosphatase that dephosphorylates acetyl CoA carboxylase

How are the processes regulated in fatty acid synthesis and oxidation?

• Fatty acid synthesis and breakdown are reciprocally regulated

• Glucagon promotes release of fatty acids from adipocytes

• High levels of fatty acyl CoA inhibit acetyl CoA carboxylase

• Malonyl-CoA made from acetyl CoA carboxylase inhibits carnitine acyl transferase I and prevents fatty acid oxidation by blocking entry into the mitochondria