Lecture 1: Central Metabolism Glycolysis - Exam 4

Question 1

What are the three primary central metabolic pathways, what’s happening in them, and when would you expect them to be used (growth on glucose, gluconate, acetate, etc.)?

Answer 1

These three pathways metabolize carbohydrates and carboxylic acids.
1) Embden-Meyerhof-Parnas Pathway (EMP pathway, or glycolysis)
2) Pentose phosphate pathway (PPP)
3) Entner-Duodoroff pathway (ED pathway)
*All of these pathways convert glucose to phosphoglyceraldehyde and then oxidized to pyruvate.

Question 2

What is the goal of the bacterial cell?

Answer 2

Self-replication

Question 3

The goal of the bacterial cell is self-replication. What does this take?

Answer 3

This takes energy-generating reactions (dissimilatory) and energy-requiring reactions (assimilatory).

Question 4

Are all dissimilatory and assimilatory reactions catalyzed in the living bacterial cell by integrated enzyme systems, with the end goal being self-replication?

Answer 4

YES!

Question 5

What is heterotrophic metabolism?

Answer 5

Biological oxidation of organic compounds, such as glucose, to yield ATP and other simpler organic or inorganic compounds which are needed by the bacterial cell for biosynthetic or assimilatory reactions.

Question 6

When does respiration take place?

Answer 6

When any organic compound is completely oxidized to CO2 and H2O.

Question 7

Aerobic respiration yields…?

Answer 7

a theoretical maximum of 38 moles of ATP from 1 mole of glucose.

Question 8

What is fermentation and what does it yield?

Answer 8

Is an incomplete form of glucose oxidation, less energy is generated. Fermentation yields 2 moles of ATP from 1 mole of glucose.

Question 9

How do all three pathways differ? How are they similar?

Answer 9

All convert glucose to phosphoglyceraldehyde (with different paths) and all oxidize 3-phosphogylceraldehyde to pyruvate (same paths).
Glycolysis yields 2 ATP, 2 NADH, 2 pyruvate
Entner-Doudoroff yields 1 ATP, 1 NADH, 1 NADPH, & 2 pyruvate
Pentose phosphate yields 1 ATP, & 2 NADPH

Question 10

What happens to NAD(P)H generated during central metabolism?

Answer 10

It goes to the ETC which drives ATP production via an ETC-generated proton gradient.

Question 11

Why is glycolysis so important for bacterial cells?

Answer 11

In addition to generating ATP and NADH, glycolysis generates intermediates that are very important in many anabolic pathways.

Question 12

What are the two phases of glycolysis? Something is converted to something else during glycolysis. What are these things? How many steps does it take?

Answer 12

Preparatory phase (steps 1-5) and the payoff phase (steps 6-10). Glucose is converted to pyruvate in 10 steps by glycolysis.

Question 13

Describe the preparatory phase.

Answer 13

-This phase is also called glucose activation phase.
-Two molecules of ATP are invested in step 1 & 3.
-Hexose chain is cleaved into two triose phosphates.
-During this, phosphorylation of glucose and its conversion to glyceraldehyde-3-phosphate take place.

Question 14

Describe the payoff phase.

Answer 14

-This phase is also called the energy extraction phase.
-Glyceraldehyde-3-phosphate is converted to pyruvate.
-ATP is formed and NAD+ is reduced to NADH.
-Because glucose is split to yield two molecules of Glyceraldehyde-3-phosphate, each step in the payoff phase occurs twice per molecule of glucose.

Question 15

What are the important enzymes? What is different about the steps these enzymes are involved in?

Answer 15

Hexokinase(step 1), Phosphofructokinase (step 3), Pyruvate kinase (step 10)
All of the steps these are involved in are irreversible.
These are also kinases, which attach phosphate groups to a substrate.
ATP is formed in the steps involving these enzymes.

Question 16

What irreversible enzymes are involved in the preparatory phase?

Answer 16

Hexokinase and phosphofructokinase.

Question 17

Describe the first step of glycolysis.

Answer 17

Glucose is phosphorylated to form glucose-6-phosphate.
-Glucose forms glucose-6-phosphate through phosphorylation via PEP in the phosphotransfer system or via hexokinase as the catalyst + ATP.
-Hexokinase is the key glycolytic enzyme here. It is a magnesium-dependent enzyme.
-The reaction is IRREVERSIBLE!

Question 18

Describe the second step of glycolysis.

Answer 18

Glucose-6-phosphate is isomerized to fructose-6-phosphate by phosphohexose isomerase.
-Phosphohexose isomerase is a magnesium dependent enzyme
-The reaction is reversible.

Question 19

Describe the third step of glycolysis.

Answer 19

Fructose-6-phosphate is further phosphorylated to fructose 1,6-biphosphate.
-The enzyme phosphofructokinase-1 catalyzes the transfer of a phosphate group from ATP to F-6-P.
-The reaction is IRREVERSIBLE.

Question 20

Describe the fourth step of glycolysis.

Answer 20

The #6 C of fructose 1,6-biphosphate is cleaved into two 3 carbon units:
-One glyceraldehyde-3-phosphate (GAP or G3P) and one molecule of dihydroxy acetone phosphate (DHAP).
-The enzyme which catalyzes the reaction is aldolase.
-The reaction is reversible.

Question 21

Describe the fifth step of glycolysis.

Answer 21

Glyceraldehyde 3-phosphate (GAP or G3P) is on the direct pathway of glycolysis, whereas DHAP is not.
-DHAP must be converted to G3P by triose-phosphate isomerase. This is necessary because G3P can be used to generate ATP.
-The net result is that glucose is now cleaved into two molecules of glyceraldehyde-3-phosphate (G3P).
-This reaction is rapid and reversible.

Question 22

Describe the sixth step of glycolysis.

Answer 22

The first step of the payoff stage is glyceraldehyde-3-phosphate oxidized to 1,3-biphosphoglycerate.
-This reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
***This is an energy-yielding reaction
-During this reaction, NAD+ is reduced to NADH.
-This is a reversible reaction.

Question 23

Describe the seventh step of glycolysis.

Answer 23

The enzyme phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-biphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.
-This is a good example of substrate-level phosphorylation.
-This reaction is reversible.

Question 24

Describe the eighth step of glycolysis.

Answer 24

3-phosphoglycerate is isomerized to 2-phosphoglycerate by shifting the phosphate group from the 3rd carbon to the 2nd carbon atom.
-The enzyme is phosphoglycerate mutase, which is magnesium dependent.
-Readily reversible reaction.

Question 25

Describe the ninth step of glycolysis.

Answer 25

2-phosphoglycerate is converted to phosphoenolpyruvate (PEP) by the enzyme enolase, which is magnesium dependent.
-One water molecule is removed.
-A high energy phosphate bond is produced.
-The reaction is reversible.

Question 26

Describe the tenth step of glycolysis.

Answer 26

PEP is dephosphorylated to pyruvate, by pyruvate kinase.
-First PEP is made into a transient intermediary of enol pyruvate, which is spontaneously isomerized into keto pyruvate, the stable form of pyruvate (resonance stabilization).
-ATP is generated during the reaction.
-Another example of substrate-level phosphorylation.
-This reaction is IRREVERSIBLE.

Question 27

The energy-yielding steps of glycolysis PER glucose produce?
What steps are oxidation reactions?
What steps are substrate-level phosphorylation?

Answer 27

2 ATP, 2 NADH, and pyruvate (pyruvate will generate more ATP in the Krebs cycle).
One oxidation reaction:
Step 6: conversion of G3P to 1,3-biphosphoglycerate with reduction of NAD+ to NADH.
Two substrate-level phosphorylation reactions:
Step 7: 1,3-biphosphoglycerate + ADP to 3-phosphoglycerate + ATP
Step 10: PEP + ADP to pyruvate + ATP

Question 28

Talk about important enzymes, what step they’re in and the energy outcome.
What’s the net yield?

Answer 28

Step 1: Hexokinase ; -1ATP
Step 3: Phosphofructokinase ; -1ATP
Step 6: G3P dehydrogenase ; +2NADH
Step 7: Phosphoglycerate kinase ; +2ATP
Step 10: Pyruvate kinase ; +2ATP
NET YIELD: 2ATP, 2NADH, and 2pyruvate

Question 29

What kind of pathway is glycolysis?

Answer 29

Glycolysis is an amphibolic pathway.
It is both a catabolic and anabolic pathway.

Question 30

In addition to generating ATP and NADH, glycolysis generates intermediates that are important in many anabolic pathways. What are some of these intermediates?

Answer 30

-Glucose-6-phosphate is a precursor to polysaccharides, pentose phosphates and aromatic amino acids.
-Fructose-6-phosphate is a precursor to amino sugars such as muramic acid and glucosamine found in the cell wall.
-3-phosphoglycerate is a precursor to the amino acids glycine, serine and cysteine.

Question 31

What is gluconeogenesis?

Answer 31

Reverse carbon flow from pyruvate to glucose

Question 32

When do microbes synthesize the glycolytic intermediates?

Answer 32

When microbes are not growing on glucose (e.g. growth on pyruvate)

Question 33

What are the three irreversible steps in gluconeogenesis?

Answer 33

1. Pyruvate kinase is not reversible.
   -It takes two enzymes for biosynthesis of PEP from pyruvate: Pyruvate carboxylase and PEP carboxykinase.
2. Phosphofructokinase is not reversible
   -Fructose 1,6-biphosphatase generates Fructose-6-phosphate
3. Hexokinase is not reversible
   -Glucose 6-phosphatase generates glucose

Question 34

Glycolytic enzymes are largely under ________ regulation.

Answer 34

Allosteric

Question 35

When does allosteric regulation occur?

Answer 35

Allosteric regulation occurs when an activator or inhibitor molecule binds at a specific regulatory site on the enzyme and induces conformational or electrostatic changes that either enhance or reduce enzyme activity.

Question 36

Discuss the regulation of glycolysis.

Answer 36

AMP, ADP, FBP, and PEP are important for regulation.
-High ADP and/or AMP levels are a signal that ATP levels are low, and this will stimulate glycolysis.
Two examples:
The activity of phosphofructokinase is allosterically inhibited by high levels of ATP or PEP and is allosterically activated by high levels of ADP or AMP (which reverse the action of ATP).
The activity of pyruvate kinase is allosterically inhibited by ATP and activated by high levels of fructose 1,6-biphosphate.