Lecture notes for Bioc462b lecture 3 - Glycolysis Jan 20, 2009

Learning Objectives

•  Explain the rationale behind the sequence of the steps in glycolysis in terms of free energy changes and ease for catalysis.

•  Describe the net energy output of glycolysis and the free energy characteristics of the steps that drive the pathway in a forward direction.

•  Identify the molecules that are needed for the continuous operation of glycolysis in addition to the enzymes and starting substrate.

•  Discuss the mechanisms of aldolase and glyceraldehydes 3-phosphate dehydrogenase and explain the roles of Schiff base and thioester in the reactions catalyzed by those enzymes. Describe how electrons are transferred to NAD+.

•  Compare the two different types of glucose transporters and understand the source of the energy used to actively transport glucose across cell membranes.

Practice homework problems: #10 and #14 (at the end of chapter 14)

Glucose plays a central role in the metabolism of carbohydrate due in large part to its low reactivity with proteins.

Triglyceride Synthesis (Glycerol moiety)

Nucleotide biosynthesis, RNA, DNA Amino Acids, Protein (through TCA cycle)

Motifs, Control, and Stability Doyle J, Csete M PLoS Biology Vol. 3, No. 11, e392

The first half of glycolysis (4 to 5 steps) generates 3 carbon intermediates that are subsequently oxidized to pyruvate during the second half (5 reactions).

Cancer cells exhibit high rates of glucose uptake and glycolysis, a phenomenon first described by Warburg (the Warburg effect). Contrary to common misconception, cancer cells do not undergo glycolysis because of a hypoxic environment. In subsequent lectures, you will see how glycolysis plays a role in biosynthesis of nucleotides, amino acids, and lipids, components required for cell growth and proliferation.

Non-metabolizable analogs are often used in uptake studies because they accumulate inside cells. Why is 6-phospho-FdG not further metabolized in glycolysis?

From Lodish et al Molecular Cell Biology, 5th Ed, Copyright © 2004 by W. H. Freeman & Company

Transporters can undergo facilitative or active transport. Transporting a substance against concentration gradient requires energy input.

Ultimately, ion gradient across membrane is powered by ATP.

The first three steps of glycolysis serve to add to phosphate groups at the ends of a six-carbon sugar (at position 1 and 6).

Check out the animation link on course website.

The aldolase reaction requires protonated Schiff base as electron sink to break a carbon-carbon bond. Activation energy of catalysis is lowered by electron delocalization leading to resonance stabilization.

The glyceraldehyde 3-phosphate dehydrogenase reaction is the first energy-yielding step in glycolysis during which electrons are transferred as a hydride ion (see last lecture) from G3P to NAD+.

The last four steps of glycolysis transfers 2 phosphate groups from a 3 carbon compound to 2 ADPs (substrate level phosphorylation).

The role of glycolysis during intense exercise can be considered as providing high energy phosphate carriers in the forms of phosphoenolpyruvate and 1,3-bisphosphoglycerate.

Electron delocalization of the products drive the large negative ∆Gs of phosphoglycerate kinase reaction and pyruvate kinase reaction.

Moving the phosphate group in 3PG from the 3 position to the 2 position facilitates the transfer of phosphate group by moving the phosphate group closer to the carboxyl group at 1 position and by coupling phosphorylysis to formation of enolpyruvate.

∆G’0 = -257 kJ/mol

Glucose → 2 pyruvate ∆G1’0 = -257 kJ/mol

2NAD+ → 2NADH ∆G2’0 = 124 kJ/mol

2ADP + 2Pi → 2ATP + 2H2O ∆G3’0 = 61 kJ/mol

Glucose + 2NAD+ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2ATP + 2H2O

∆G1’0 + ∆G2’0 + ∆G3’0 = -72 kJ/mol

Overall, -72 kJ/mol of free energy is “lost” as cost of driving glycolysis in a forward direction.

Copyright 2001, Lawrence Chasin and Deborah Mowshowitz Department of Biological Sciences, Columbia University, New York

Discrepancy between standard state ∆G and actual ∆G of individual steps shows the two phosphorylation steps in the second half of glycolysis as the steps that make glycolysis irreversible in tissues that turnover ATP quickly. Actual ∆G is can be different under different physiological conditions like running or sitting still.