Question

Discuss the following in details:

ATP Synthase. Start from the production of malonyl CoA in the cytosol and end with the product of butyrylCoA.

Answer 1

ATP is synthesized from its precursor, ADP, by ATP synthases. These enzymes are found in the cristae and the inner membrane of mitochondria, the thylakoid membrane of chloroplasts, and the plasma membrane of bacteria . Usually, there is a general understanding that ATP generation occurs in mitochondria. However, in the case of bacteria and archaea that lack mitochondria, ATP synthase is found in their plasma membrane. Additionally, ATP synthases are licensed to inhabit the chloroplast of plant cells. The structure and procedure of ATP synthesis is similar in all three locations except that light energy excites electrons enabling transmembrane movement of H_⁺ ions in chloroplasts. The general nomenclature of ATP synthase as F_oF₁ changes to CF_oCF₁ for chloroplast ATP synthase and ECF_oECF₁ for Escherichia coli’s ATP synthase.

The enzymes of fatty acid synthesis are packaged together in a complex called as fatty acid synthase (FAS). • The product of FAS action is palmitic acid. (16:0). • Modifications of this primary FA leads to other longer (and shorter) FA and unsaturated FA. • The fatty acid molecule is synthesized 2 carbons at a time • FA synthesis begins from the methyl end and proceeds toward the carboxylic acid end. Thus, C16 and C15 are added first and C2 and C1 are added last. • C15 and C16 are derived directly from acetylCoA. For further step-wise 2-carbon extensions, acetylCoA is first activated to malonyl CoA, a 3-carbon compound, by the addition of a CO2.

Citrate Shuttle • FAs are synthesized in the cytoplasm from acetylCoA • AcetylCoA generated from pyruvate by the action of PDH and by β-oxidation of fatty acids is in the mitochondria. • For fatty acid biosynthesis, acetylCoA has to be transported from the mitochondria to the cytoplasm. This is done via a shuttle system called the Citrate Shuttle. • AcetylCoA reacts with oxaloacetate to give citrate. A tricarboxylate translocase transports citrate from mitochondria to cytosol. • In the cytosol, citrate is cleaved back to oxaloacetate and acetylCoA. This reaction is catalyzed by ATP-citrate lyase and requires the hydrolysis of one molecule of ATP.

Citrate Shuttle (regeneration of pyruvate) • Oxaloacetate is converted back to pyruvate for re-entry into mitochondria • Step 1. Oxaloacetate + NADH + H+ Æ malate + NAD+. Reverse of the TA cycle reaction. Catalyzed by cytosolic malate dehydrogenase. • Step 2. Malate + NADP+ Æ pyruvate + CO2 + NADPH. Catalyzed by malic enzyme • Pyruvate translocase transports pyruvate into mitochondria. • Pyruvate is converted to oxaloacetate by pyruvate carboxylase with coupled hydrolysis of one ATP. Pyruvate + ATP + CO2 + H2O Æ oxaloacetate + ADP + Pi (reaction of gluconeogenesis) • Net Reaction: NADP+ + NADH + H+ + ATP + H2O Æ NADPH + NAD+ + ADP + Pi • Thus, transport of acetylCoA to cytosol requires expense of one ATP and conversion of one NADH to NADPH.

MalonylCoA • Malonyl CoA is synthesized by the action of acetylCoA carboxylase. Biotin is a required cofactor. • CH3COSCoA + CO2 + ATP Æ OOC-CH2-COSCoA + ADP +Pi (enzyme: acetylCoA carboxylase) • This is an irreversible reaction. AcetylCoA carboxylation is a rate-limiting step of FA biosynthesis. • AcetylCoA carboxylase is under allosteric regulation. Citrate is a positive effector and palmitoyl CoA is a negative effector.

ATP Synthase • ATP Synthase is a polypeptide chain with multiple domains, each with distinct enzyme activities required for ATP biosynthesis. • ACP: Recall that CoA is used as an activator for βoxidation. For ATP biosynthesis, the activator is a protein called the acyl carrier protein (ACP). It is part of the ATP Synthase complex. The acyl groups get anchored to the CoA group of ACP by a thioester linkage • Condensing enzyme/β-ketoacyl synthase (K-SH). Also part of ATP Synthase, CE has a cysteine SH that participates in thioester linkage with the carboxylate group of the ATP molecule. • During ATP biosynthesis, the growing fatty acid chain alternates between K-SH and ACP-SH

Step-wise reactions 1. The acetyl group gets transferred from CoA to ACP by acetyl CoA-ACP transacylase. 2. The acetyl (acyl) group next gets transferred to the K arm of ATP Synthase complex. 3. Next, the malonyl group gets transferred from CoA to ACP by malonyl CoA ACP transacylase. This results in both arms of ATP Synthase occupied forming acylmalonyl-ACP. 4. The COO group of malonyl ACP is removed as CO2, the acetyl group (C16 and C15) gets transferred to the alpha carbon of malonyl ACP. This results in 3- keto acyl ACP. • The 3-keto group is converted to a CH2 by a series of reactions reverse to ATP β-oxidation. Namely, 1. reduction to hydroxyl group. Enz: 3-keto acyl ACP reductase 2. dehydration to form a 2,3 double bond and Enz: 3- hydroxy acyl ACP dehydratase 3. a second reduction to remove the double bond. Enz: Enoyl ACP reductase • Both reduction reactions require the reduced cofactor NADPH. This is generated by the hexose monophosphate pathway of phosphogluconate pathway and during the citrate shuttle. • The result of the first cycle of fatty acid biosynthesis is a four carbon chain associated to the ACP arm. • This chain gets transferred to the K arm. • A new malonyl CoA is introduced on the ACP arm. • The reactions proceed as before. For each cycle the acyl group transferred to the α-carbon of malonyl CoA is 2- carbons longer the previous cycle. • At the end of 7 cycles a 16 carbon chain is attached to the ACP arm (palmitoyl ACP). • The C16 unit is hydrolyzed from ACP yielding free palmitate • Net reaction: Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ Æ Palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6H2O.