Pentose Phosphate Pathway
- Understand how the pentose phosphate pathway produces NADPH for biosynthetic processes.
The pentose phosphate pathway comprises two sets of reactions: Oxidative and Non-Oxidative
In the Oxidative pathway, glucose-6-phosphate is converted into ribulose-5-phosphate and two NADPH are produced in the process.
In the non-oxidative process, 2C5 molecules are first converted to Glyceraldehyde-3phosphate (C3) and a C7 using transketolase with thiamin pyrophosphate as a cofactor. The glyceraldehyde and the C7 are then converted into fructose-6-phosphate (C6) and a C4 molecule using tranaldolase. Subsequently, the C4 and a C5 molecule form a Fructose-6-phosphate and a Glyceraldehyde 3 phosphate again using transketolase.
Net reaction: 3C6--> 3C5--> 2C6 +C3 + 6NADPH
- Know the chemical role of NADPH and the pathways that use it in the human body.
NADPH is a reducing agent that protects cells from oxidative damage from free radicals. NADPH is also important for the production of fatty acids and steroids in the cell. It is primarily produced by the pentose phosphate pathway. It can also be produced by the decarboxylation of OAA to produce pyruvate in the mitochondria.
3. Understand how the pentose phosphate pathway produces products other than NADPH that are essential for nucleic acid synthesis and can be shuttled into glycolysis.
4. Realize that the utilization of glucose-6-phosphate in the pentose phosphate pathway varies with the cell’s need for ATP, NADPH and ribose-5-phosphate.
5. Recognize that bodily needs will determine whether a portion or the whole pathway is employed.
Fatty Acid Biosynthesis
1. Describe the committed step in fatty acid synthesis; i.e. the substrates, products,
enzymes and coenzymes and the regulatory mechanism.
The first and irreversible step of fatty acid synthesis involves the carboxylation of Acetyl Co-A to produce Malony Co-A. This step requires Acetyl CoA carboxylase, ATP, and bicarbonate. Acetyl CoA carboxylase is activated by citrate and deactived by Malonyl CoA and Palmitoyl CoA. In addition, a glucagon activated protein kinase can PHOSPHORYLATE it to INACTIVATE it and an insulin activated dephosphorylase can ACTIVATE it by DE-phosphorylating it. On a third level, a high carbohydrate diet will increase enzyme expression, whereas a low carb diet will reduce enzyme expression.
2. Explain the elongation reactions of fatty acid synthesis and the role of malonyl
In the example of palmitate, 8 C2 residues will make the 16:0 fatty acid. The growing fatty acid chain is joined to a phosphopantethiene group attached to a serine residue in the synthase. 2 molecules of NADPH are used for every C2 addition. The role of NADPH is to reduce the carbonyl group that comes with the C2. In the first step, it is reduced to a hydroxyl group. Subsequently, after the dehydration step, the NADPH removes the double bond right before the chain under construction is transferred to the the cystein residue of the synthase.
3. Calculate the energy cost of the synthesis of a particular fatty acid and understand
the origins of the NADPH used.
2 NADPH are used per C2 added. So, 14 NADPH are used for palmitate. When you take the malonyl CoA production into consideration, the reaction looks as shown below.
'8 acetyl-CoA + 14 NADPH + 7 ATP' à 'palmitate + 14 NADP'+' + 8 CoA + 7 ADP + 7 P'i
4. Understand the interrelationship between glucose metabolism and fatty acid
synthesis with an emphasis on the shuttle systems.
Fatty acid synthesis occurs in the cytoplasm but the Acetyl-CoA required for it is produced in the mitochondrion. It is transported back into the cytosol using a shuttle mechanism involving citrate. OAA and Acetyl CoA form Citrate using citrate synthase and the citrate travels to the cytosol where it is cleaved to re-form OAA and Acetyl Co-A.
5. Outline the elongations and desaturations that can occur on fatty acids.
Desaturations are first introduced at the 9-10 position. If that’s not possible due to an extant double bond, the double bond is then placed at the 6-7 position. Also, desaturations can never be introduced beyond the 9-10 position, but they can be moved there by successive C2 unit additions.
- Understand why linoleate and linolenate are essential fatty acids.
Linoleate and Linolenate are essential fatty acids because mammals cannot introduce double bonds beyond C-9 and we must, therefore, obtain them from other sources. These fatty acids form the precursors of physiologically important molecules like Prostaglandins (Muscle contraction/relaxation and platelet aggregation inhibition), Thromboxanes (platelet aggregation), and Leukotrienes (vasoconstriction and bronchoconstrictors).
7. Understand the role of COX-1 and COX-2 in eicosanoid metabolism and their
relevance as drug targets.
Cyclooxygenases (COX) are required for the cyclization of Eicosinoids to form, for example, prostaglandins from Arachidonic acid. COX 1 is required for the health of the stomach and is blocked by NSAIDS along with COX2. This can result in stomach bleeding and kidney damage. Aspirin also blocks COX to elicit most of its effects like reduction of inflammation and the prevention of blood clotting. COX2 is the bad cyclooxygenase. NSAID use in the long run can reduce the risk of colon cancer since COX 2 can serve as a tumour promoter.