UCL BENG0004 2021 Lecture 17: Pentose Phosphate Pathway PDF

BENG0004 Biochemistry and Molecular Biology Emily Kostas Lecture 17 Pentose Phosphate Pathway  The Pentose Phosphate Pathway An anabolic pathway that generates NADPH reducing power for biosynthetic reactions. It also builds the starting c...

BENG0004 Biochemistry and Molecular Biology Emily Kostas Lecture 17 Pentose Phosphate Pathway  The Pentose Phosphate Pathway An anabolic pathway that generates NADPH reducing power for biosynthetic reactions. It also builds the starting compounds for making aromatic amino acids and nucleotides. Erythrose-4-phosphate is made for aromatic amino acid biosynthesis Ribose-5-phosphate is made for nucleotide biosynthesis NADPH is the reduced nicotinamide adenine dinucleotide co-factor used in cells for building compounds – anabolic reactions. The cell separates the catabolic pathways and the anabolic pathways chemically by using NADH for catabolism and NADPH for anabolism. This means that the two types of metabolism do not compete for the same pool of nucleotides. An overview of metabolism – catabolism and anabolism ATP NADPH NADH NADP NAD+  The cofactors NAD+, NADH, NADP and NADPH Chemical bonds carry energy stored in the electrons that two atoms share. These cofactors can take energy in the form of electrons from one molecule and use it elsewhere in the cell. Most of the NADH that is made in catabolic pathways such as glycolysis and fatty acid oxidation is converted back to NAD+ at the electron transport chain where it is used to make a proton gradient and that energy is used to make ATP. There are pathways in the cell that specifically use NADP as the oxidant and the NADPH generated is used in anabolic reactions. The cofactors NAD+, NADH, NADP and NADPH  Nicotine Addictive. Binds to a class of acetyl choline receptors. When an enzyme such as a dehydrogenase uses NAD+ in a reaction it is removing 2 electrons and a proton. The electrons carry the energy of the original bond. So the NADH is a universal way of carrying energy around the cell as well as the NADH being able to be used as a reducing agent in other reactions. We can monitor dehydrogenase reactions that use NAD+/NADH by the increase in absorbance of the NADH at 340 nm in a spectrophotometer. The major catabolic pathways Glycolysis The pentose phosphate pathway b-oxidation The pentose phosphate pathway For anabolic reactions For nucleotide biosynthesis For aromatic amino acid biosynthesis Glycolysis Pentose phosphate pathway  Oxidative phase of the pentose phosphate pathway Glucose -6-phosphate Gluconolactonase 6-phosphogluconate dehydrogenase dehydrogenase Compounds 1 Glucose-6-phosphate 2 6-phosphogluconolactone 3 6-phosphogluconate 4 Ribulose-5-phosphate  The non-oxidative phase of the pentose phosphate pathway isomerase Ribose-5-phosphate Sedoheptulose-7-phosphate Erythrose-4-phosphate Xylulose-5-phosphate transketolase transaldolase transketolase epimerase Ribulose-5-phosphate Glycerinaldehyde-3- phosphate Xylulose-5-phosphate Glycerinaldehyde-3- Fructose-6-phosphate phosphate The enzymes transketolase and transaldolase interchange 2-carbon (TK) and 3-carbon (TA) units between the sugar-phosphates and are all freely reversible. Overview of the two phases of the pentose phosphate pathway Glycolysis Pentose Phosphate Pathway Cells that highly utilise the pentose phosphate pathway and NADPH 1) The liver, adipose tissue, adrenal cortex, testis and lactating mammary gland have high levels of the PPP enzymes. The reactions of fatty acid biosynthesis and steroid biosynthesis utilize large amounts of NADPH. About 30% of the oxidation of glucose in the liver occurs via the PPP. 2) Additionally, erythrocytes (red blood cells) utilize the reactions of the PPP to generate large amounts of NADPH used in the reduction of glutathione which protects erythrocyte proteins from oxidation damage. 3) The conversion of ribonucleotides to deoxyribonucleotides (through the action of ribonucleotide reductase) requires NADPH as the electron source, therefore, any rapidly proliferating cell needs large quantities of NADPH. 4) Although the PPP operates in all cells, with high levels of expression in the above indicated tissues, the highest levels of PPP enzymes (in particular glucose 6-phosphate dehydrogenase) are found in neutrophils and macrophages. These leukocytes are the phagocytic cells of the immune system and they utilize NADPH to generate superoxide radicals from molecular oxygen in a reaction catalyzed by NADPH oxidase. Superoxide anion, in turn, serves to generate other reactive oxygen species (ROS) that kill the phagocytised microorganisms. Flux through the glycolytic and pentose phosphate pathways Cancer cells exhibit the Warburg effect where low oxygen respiration is seen and there is high flux through the PPP and to fermentation products An example of flux diagrams used to demonstrate the flow (flux) of carbon from glucose or galactose as the carbon source in E. coli Corynebacterium glutamicum flux diagram PPP = pentose phosphate pathway. EMP = Embden-Meyerhof-Parnas glycolytic pathway. TCA = tri- carboxyic acid. ANA = anaplerotic pathways Diagram of alterations in flux through carbon pathways in a recombinant lysine over producer. Numbers are % carbon from input. Biochemistry Berg et al 8th Edition Pentose phosphate pathway pages 601 (20.3) – 606. 608