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NS 315Unit 4: Carbohydrate Metabolism
Jeanette Andrade MS,RD,LDN,CDE
Kaplan University
Objectives
We want to learn about: Glycolysis and ATP formation
Understand Gluconeogenesis, when, where and how
Krebs Cycle and Electron Transport Chain
Definitions
Glycolysis: central pathway for the catabolism of carbohydrates; occurs in most organs
Gluconeogenesis: Biosynthesis of new glucose; occurs mainly in liver
Krebs cycle- series of enzymatic reactions in aerobic organisms involving oxidative metabolism of acetyl units and producing high-energy phosphate compounds, which serve as the main source of cellular energy
Electron Transport Chain (ETC)- Composed of mitochondrial enzymes that transfers electrons from one transport to another, resulting in the driving force for the formation of ATP
Oxidative phosphorylation- Process occurring in the cell, which produce energy and synthesizes ATP
Definitions
Pyruvate: final 3 carbon molecule of glycolysis, involved in the Krebs cycle which facilitates energy production
Adenosine diphosphate/Adenosine triphosphate: energy storing molecule used by an organism on a daily basis
NAD/NADPH: Reducing agent in several anabolic reactions such as lipid and nucleic acid
FAD/FADH: Reducing agent in several anabolic reactions such as lipid
Aerobic: in the presence of oxygen
Anaerobic: no presence of oxygen
Glycolysis Animation
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html
Fates of PyruvateUnder aerobic conditions
Under anaerobic conditions
In most aerobic organisms, pyruvate continues in the formation of Acetyl CoA and NADH that follows into the Krebs cycle and
Under anaerobic conditions, such as during exercise or in red blood cells (no mitochondria), pyruvate is reduced to lactate by lactate dehydrogenase producing NAD for glycolysis
Pathways during Glycolysis
Aerobic- with oxygen
The main energy releasing pathway in most human cells
Continues in the mitochondrion where oxygen serves as the final electron acceptor
1 glucose + 6 oxygen 6 carbon dioxide +6 water
36 or 38 ATPs are produced (total after all cycles: glycolysis, krebs and ETC)
Anaerobic- without oxygen
Fermentation pathway and anaerobic electron transport- many bacteria and humans, when oxygen is limited, use this pathway
Ends in the cytoplasm where other substances besides oxygen is the final electron receptor
Only 2 ATP are produced
Gluconeogenesis
During starvation (not eating for 16 hours plus), the brain can use ketone bodies for energy by converting to Acetyl CoA, usually gluconeogenesis creates glucose when glycogen stores are depleted
Synthesis of glucose from 3-4 carbon precursors is a reversal of glycolysis
2 pyruvate + 2 NADH + 4 ATP + 2 GTP glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi
Gluconeogenesis
3 reactions in glycolysis are essentially irreversible, thus they are bypassed in gluconeogenesis: Hexokinase (1) Phosphofructokinase (3) Pyruvate Kinase (10)
Share 7 of the 10 steps in glycolysis
Fed state
Cytoplasm
All cells
Fasting state
Cytoplasm
Liver mostly, but also kidney
Glycolysis vs Gluconeogenesis
Activation of Pyruvate
Before the Kreb’s cycle begins, pyruvate must be activated into acetyl CoA
Pyruvate dehydrogenase complex (PDHC) is an enzyme that catalyzes the oxidative decarboxylation of pyruvate to acetyl CoA
PDHC is a multi-enzyme comprising of 5 coenzymes- which include many vitamins such as thiamin (thiamin pyrophosphate), riboflavin (FAD), and niacin (NAD)
Krebs Cycle
Also known as the citric acid cycle or tricarboxylic acid (TCA) cycle
Under aerobic conditions pyruvate enters the mitochondria MATRIX and is oxidized to Acetyl CoA which enters the Krebs cycle
Krebs cycle can occur after glycolysis, after Beta oxidation or protein degradation to provide energy for cellular respiration
Equation for Krebs cycle with the beginning products and the ending. 8 steps involved
2 pyruvate + 2 GDP + 2 H3PO4 + 4 H2O + 2 FAD + 8 NAD+ ----> 6 CO2 + 2 GTP + 2 FADH2 + 8 NADH
Krebs Cycle
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_the_krebs_cycle_works__quiz_1_.html
Summary TCA
Occurs in the mitochondrial matrix
Uses acetyl CoA to produce: 3 NADH, 1 FADH, 1 GTP, 2CO2
Produce intermediates for biosynthetic pathways such as aminoacid synthesis, gluconeogenesis, pyrimidine synthesis, phorphyrin synthesis, fatty acid synthesis, isoprenoid synthesis.
Electron Transport Chain (ETC)
Final pathway by which electrons generated from oxidation of carbs, protein and fatty acids, are ultimately transferred to O2 to produce H20
Located in the inner mitochondrial membrane Electrons travel down the chain, pumping protons into the
intermembrane space creating the driving force to produce ATP in a process called oxidative phosphorylation
There are 4 complexes that comprise the ETC
Electron Transport Chain
http://vcell.ndsu.edu/animations/etc/movie.htm
Summary ETC
Reduced electron carriers NADH & FADH2 reduce O2 to H2O via the ETC. The energy released creates a proton gradient across the inner mitochondrial membrane. The protons flow down this concentration gradient back across the inner mitochondrial membrane through the ATP Synthase. The driven force makes this enzyme rotate and this conformation generates enough energy to make ATP.
Oxidation of NADH to NAD+ pumps 3 protons which charges the electrochemical gradient with enough potential to generate 3 ATPs.
Oxidation of FADH2 to FAD+ pumps 2 protons which charges the electrochemical gradient with enough potential to generate 2 ATPs.