Lecture 25 - Glycolysis

What are isoenzymes (isozymes)?

"Isoenzymes" or "isozymes": enzymes that catalyze the same reaction but have different kinetic (Km) and regulatory properties •Different isozymes have similar but non-identical amino acid sequence •Different isoenzymes can occur in different organs or tissues

What are the substrates, products, enzymes, and special characters for each reaction?

1. hexokinase 2. glucophosphate isomerase, Make the C1 available for phosphorylation, prepared for cleavage. 3.Phosphofructokinase = PFK, commitment step, cost 1 atp 4. Aldolase, pull rxn towards cleavage 5. triose phosphate isomerase

Which step is a redox reaction?

Step 6. Oxidative phosphorylation of glyceraldehyde 3-phosphate

How is NAD+ regenerated for glycolysis through lactate and alcohol fermentation? What are the enzymes involved?

lactate dehydrogenase, pyruvate decarboxylase, alcohol dehydrogenase. pyruvate and lactate take h fromnadh

What is glycolysis "payoff" phase?

oxidative conversion of glyceraldehyde 3 phosphate to pyruvate and the coupled formation of ATP and NADH

What is glycolysis "preparation" phase?

phosphorylation of glucose and its conversion to glyceraldehyde 3 phosphate OR In this phase, a glucose molecule is converted to two triose phosphate molecules, at the expense of (driven by) the hydrolysis of two ATP molecules.

What are the five reactions in the prep phase?

1. formation of glucose 6 phosphate 2. isomerization 3. second phosphorylation 4. aldol cleavage 5. isomerization

What are the basics of glycolysis?

-first stage of glucose metabolism -incomplete glucose oxidation -in cytoplasm -one glucose splits to 2 pyruvate (3 carbon) -no carbon released as CO2 -2 nad+ becomes 2nadh -2 atp made

How many redox reactions are there in glycolysis?

1

What are the two types of hexokinase isozymes? How are they different?

1)Hexokinases I-III: Low Km: 0.1 mM or 1 x 10-4 M, located in all tissues except Liver. 2)Glucokinase: High Km: 10 mM or 1 x 10-2 M, located primarily in Liver (pancreas, small intestine, and brain also have some).

How many priming reactions are there in glycolysis?

2

How many substrate level phosphorylation reactions are there in glycolysis?

2

How does fructose-2,6-biosphosphate regulate phosphofructokinase?

26bpp activates enzyme

What are the five reactions in the payoff phase?

6. Oxidative phosphorylation of glyceraldehyde 3-phosphate 7. 1,3-bisphosphoglycerate: The first substrate-level ADP phosphorylation 8. Conversion of 3-phosphoglycerate to 2-phosphoglycerate 9. Dehydration of 2-phosphoglycerate 10. The second substrate-level phosphorylation

What are the substrates, products, enzymes, and special characters for each reaction? payoff

6. glyceraldehyde 3-phosphate dehydrogenaseThe only redox reaction in glycolysis. Generates 2 NADH (per glucose). The phosphorylation uses inorganic phosphate 7.phosphoglycerate kinase1) Relief of repulsion 2) Resonance stabilization of two acids 3) Increased entropy 8.phosphoglycerate mutase 9. enolase 10.pyruvate kinase

What is substrate-level phosphorylation?

Formation of ATP by phosphoryl- group transfer from a substrate (as distinguished from ATP synthesis via respiration-linked phosphorylation).

What is hexokinase?

Hexokinases are used to keep glucose in the cell: enzymes introducing PO4 on glucose, using ATP. There are 4 hexokinases (isozymes) but only 2 types important for our discussion 1) Hexokinases I-III - a family that have low Km's for glucose

Is hexokinase inhibited by glucose-6-phosophate in liver?

Liver only has Glucokinase! (no Hexokinase I-III). Why?

Why metabolic pathways need to be regulated?

Metabolic flux must be controlled: •Demand changes: The demand for ATP production in muscle may increase 100-fold in a few seconds in response to exercise. •Supply changes: Relative proportions of carbohydrate, fat and protein in the diet vary from meal to meal. The supply of fuels obtained in the diet is intermittent (between meals, starvation). Enzyme regulation is needed: •To control the rate of the respective pathway, the cells turn on or shut off regulatory enzymes in glycolysis.

Where does the energy come from for ATP synthesis in substrate level phosphorylation?

The second substrate-level phosphorylation

Does the "standard free energy change" (standard delta G) reflect the "real free energy change" in the cells (delta G)?

The standard free energy changes (ΔG'o) of some of the steps of glycolysis are positive, even though the overall standard free energy is negative.

What are the control points in glycolysis?

Three reactions exhibit large negative DG's; the enzymes that catalyze these reactions are sites of allosteric control •Hexokinase •Phosphofructokinase •Pyruvate kinase

What is a priming reaction?

add phosphate

How do other monosaccharides enter glycolysis? (in general, no details)

conversion to different substrates

Why does NAD+ need to be regenerated for glycolysis?

to allow glycolysis to continue, both processes do this

Why we say that phosphofructokinase regulation is complex?

•ATP is a required substrate, but at high concentration, ATP can also bind to an allosteric site and inhibit PFK. •This inhibition is relieved by AMP, which competes with ATP for binding at the allosteric site. •This enzyme is also inhibited by citrate and activated by F 2,6-BP

What are the chemical logics in glycolysis?

•Glycolysis achieves three important things: •energy production (ATP and NADH) •precursor production (3-phosphoglycerate feeds into other pathways) •breakdown of glucose into pyruvate to feed the TCA cycle •Glucose is split into two identical trioses (after isomerization): •simplifies metabolism, since only one set of enzymes needed downstream •Phosphorylation traps intermediates inside the cell, and prepare for phosphoryl group transfer in the payoff phase (which releases energy). •Isomerization positions carbonyl (glucose to fructose ) to make the C1 available for phosphorylation, prepare for cleavage into two triose molecules.