Biochem Exam 3 Part 5

Name five facts about glucose.

• stored in polymers

• readily released

• travels easily in blood

• made from simple precursors (gluconeogenesis)

• will not release energy until broken

What types of cells in glycolysis common in?

Trick question. It’s common in all cells (prokaryotic and eukaryotic)

What is the first stage of glucose metabolism in organisms?

glycolysis (central metabolic pathway)

What part of the cell does glycolysis occur in?

cytoplasm

Glycolysis plays a key role in the way organisms _________________ from nutrients.

extract energy

What does glycolysis produce?

2 pyruvates, 2 ATP, 2 NADH (high energy electron carrier)

Does glycolysis require oxygen?

It occurs with or without. None of the steps require oxygen.

What are the two stages of glycolysis?

stage one- traps glucose in the cell and modifies it so that it can be cleaved into a pair of phosphorylated 3-carbon compounds (steps 1-5)

stage two- oxidizes the 3-carbon compounds to pyruvate while generating two molecules of ATP (steps 6-10)

Starting stage 2, all reactions are doubled (only the path from one glyceraldehyde-3-phosphate is usually shown)

Describe this diagram.

This shows the stages of glycolysis. Reactions are doubled after stage two.

What are the first five reactions of glycolysis?

Step 1: Phosphorylation of glucose to give glucose-6-phosphate (-16.7 kJ/mol) hexokinase

Step 2: Isomerization of glucose-6-phosphate to give fructose-6-phosphate (+1.7 kJ/mol) phosphoglucose isomerase

Step 3: Phosphorylation of fructose-6-phosphate to yield fructose-1,6-bisphosphate (-14.2 kJ/mol) phosphofructosekinase (PFK)

Step 4: Cleavage of fructose-1,6-bisphosphate to give two 3-carbon fragments, glyceraldehyde-3- phosphate and dihydroxyacetone phosphate (+23.8 kJ/mol) aldolase

Step 5: Isomerization of dihydroxyacetone phosphate to give glyceraldehyde-3-phosphate (+7.5 kJ/mol) triose phosphate isomerase

What are the last five reactions of glycolysis?

Step 6: Oxidation coupled phosphorylation of glyceraldehyde-3-phosphate to give 1,3- bisphosphoglycerate (+6.3 kJ/mol) glyceraldehyde 3-phosphate dehydrogenase

Step 7: Transfer of a phosphate group from 1,3- bisphosphoglycerate to ADP to give 3- phosphoglycerate (-18.8 kJ/mol) phosphoglycerate kinase

Step 8: Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate (4.6 kJ/mol) phosphoglycerate mutase

Step 9: Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate (+1.7 kJ/mol) enolase

Step 10: Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate (-31.4 kJ/mol) pyruvate kinase

What happens when glucose enters the cell through a specific transport protein?

it is phosphorylated at the expense of ATP to form glucose 6- phosphate

What catalyzes the reaction that phosphorylates glucose?

Hexokinase (requires Mg2+ or Mn2+ as cofactor) - required for stabilization of triosephosphate of ATP)

What does hexokinase employ to minimize hydrolysis of ATP?

substrate-binding induced fit

What has to happen to the enzyme for the CH2OH of glucose to attack ATP and cause its hydrolysis?

the enzyme has to be rigid

What does this diagram show evidence of?

hexokinase being able to phosphorylate a variety of hexoses

Why is phosphorylation endergonic?

it is driven by the free energy of hydrolysis of ATP (regulated step of glycolysis)

Is hexokinase specific to glucose?

No. Any hexose can be the substrate.

Does hexokinase or glucokinase have a higher Km?

glucokinase

What is hexokinase inhibited by?

the G6P product

The two lobes of hexokinase are separated in the absence of ___________ (left). The conformation of hexokinase changes markedly on binding ____________ (right).

glucose, glucose

The addition of a _____________________ to glucose effectively traps it in the cell.

phosphate group

What decrease the concentration of free glucose, thus favoring additional import of the molecule?

phosphorylation

Liver has ___________________, which has a much _________ Km (lower affinity) for glucose.

glucokinase, higher

When is glucose converted to G6P in the liver?

when glucose concentration gets high and the liver is able to release glucose into the blood for the body to use

What does phosphoglucose isomerase catalyze?

the conversion of glucose 6-phosphate to fructose 6-phosphate (readily reversible)

What does the addition of a second phosphate to form fructose 1,6-biphosphate cause?

it causes the carbohydrate to be trapped in the fructose form (irreversible reaction catalyzed by allosteric enzyme phosphofructokinase (PFK)

phosphofructokinase (PFK)

the most important regulatory enzyme of glycolysis - committed step/rate-limiting step

Out of glucose-6-phosphate, fructose-6-phosphate, and 1,6-biphosphate, which cannot play roles in other pathways?

1,6-biphosphate

What formation commands that a molecule must undergo the rest of the reaction of glycolysis?

formation of fructose-1,6-biphosphosphate

Describe this diagram.

This shows the aldolase reaction (step 4 of glycolysis).

Is the aldolase reaction exergonic or endergonic?

endergonic under standard conditions

HOWEVER

ΔG is negative in intracellular conditions (actual concentration of reactants + products)

this is why it’s important to consider condition in the cell ΔG, rather than ΔG ͦ ‘ when deciding in which direction a reaction is favored

Which one can be processed to pyruvate to yield ATP: glyceraldehyde-3 phosphate (GAP) or dihydroxyacetone phosphate (DHAP)

glyceraldehyde-3 phosphate (GAP)

What allows GAP to be further metabolized?

the enzyme triose phosphate isomerase interconverting DHAP to GAP

Triose phosphate isomerase deficiency is lethal. What can it be characterized by?

severe hemolytic anemia and neurodegeneration

Are both of the isomerization reactions in glycolysis exergonic or endergonic?

endergonic (can occur within cells because the products are immediately removed by the next step in the pathway)

The oxidation of what powers the formation of a compound having high phosphoryl-transfer potential?

aldehyde

The oxidation of _____ generates 1,3-biphosphoglycerate in a reaction catalyzed by __________________________________________________________________.

GAP, glyceraldehyde 3 -phosphate dehydrogenase

What are the two steps in the formation of 1,3-biphosphoglycerate?

1. highly exergonic oxidation of carbon 1 in GAP to an acid

2. highly endergonic formation of 1,3-biphosphoglycerate from the acid

What are the two steps of 1,3-biphosphoglycerate formation linked by?

formation of energy-rich thioester in the active site of glyceraldehyde 3 -phosphate dehydrogenase

What is unique about the formation of 1,3-biphosphoglycerate?

this is the generation of the FIRST ENERGY RICH compound

Describe this diagram.

This shows the standard free energies of hydrolysis of some phosphorylated compounds

Describe this diagram.

This shows oxidation reaction and acyl-phosphate reaction (step 6). Inorganic phosphate is an essential substrate in this reaction.

Describe this diagram.

This shows the free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation (1st part).

Describe this diagram.

This shows the free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation (1st part).

The favorable oxidation (of aldehyde to a carboxylic acid) and unfavorable phosphorylation are coupled by the thioester intermediate (energy rich intermediate)

substrate phosphorylation

the energy of oxidation of the carbon atom, initially trapped as 1,3-biphosphateglycerate, is used to form ATP

Does ATP or 1,3-bisphosphoglycerate have a greater phosphoryl transfer potential?

1,3-bisphospoglycerate so can be used to power synthesis of ATP from ADP and Pi in reaction catalyzed by phosphoglycerate kinase

Describe this diagram.

This shows the phosphoglycerate kinase reaction (reaction 7)

First Substrate level phosphorylation - high energy molecule (1,3-BPG in this reaction)) adds phosphate to ADP to generate ATP

What is reaction 8 of glycolysis?

3-Phosphoglycerate is converted into 2-phosphoglycerate by phosphoglycerate mutase

What is reaction 9 of glycolysis?

A dehydration reaction, catalyzed by enolase, produces phosphoenolpyruvate (PEP)

Why is phosphoenolpyruvate a high phosphoryl-transfer compound?

because of the presence of the phosphate traps the compound in the unstable enol tautomer

What is generated when ADP is phosphorylated at the expense of PEP, and what is this reaction catalyzed by (reaction 10)?

it generates ATP and pyruvate, and this reaction is catalyzed by pyruvate kinase

Describe this diagram.

This shows 3-phosphoglycerate being converted into 2-phosphoglycerate by phosphoglycerate mutase (reaction 8)

*SOURCE OF 2,3 BPG

mutase

enzyme that moves a functional group, such as a phosphate, to a new location in a substrate molecule

Describe this diagram.

2-phosphoglycerate molecule loses one molecule of water, producing phosphoenolpyruvate

Describe this diagram.

This shows the second substrate-level phosphorylation (step 10- pyruvate kinase reactions)

What does this diagram show?

This is the net reaction for glycolysis (exergonic).

overall favorability of the pathway that helps to drive forward those reactions that are somewhat endergonic

Describe this table.

This shows the reactions of glycolysis

What are the reactions that require ATP in glycolysis?

• phosphorylation of glucose to give glucose-6-phosphate

• phosphorylation of fructose-6-phosphate to give fructose-1,6- bisphosphate

What are the reactions that produce ATP in glycolysis?

• transfer of phosphate group from 1,3-bisphosphoglycerate to ADP

• transfer of phosphate group from phosphoenolpyruvate to ADP

Which enzymes catalyze reactions that require ATP in glycolysis?

• hexokinase

• glucokinase

• phosphofructokinase

Which enzymes catalyze reactions that produce ATP in glycolysis?

• phosphoglycerate kinase

• pyruvate kinase

What are the three irreversible steps in glycolysis (control points) catalyzed by?

• hexokinase

• phosphofructokinase

• pyruvate kinase

Why is the regeneration of NAD+ crucial to fermentations?

• The conversion of glucose into pyruvate generates ATP, but for ATP synthesis to continue, NADH must be reoxidized to NAD+. This vital coenzyme is derived from the vitamin niacin (B3).

• NAD+ can be regenerated by further oxidation of pyruvate to CO2, or by the formation of ethanol or lactate from pyruvate.

Describe this diagram.

This shows the diverse fates of pyruvate.

Ethanol and lactate can be formed by reactions that include NADH. Alternatively, a two-carbon unit from pyruvate can be coupled to coenzyme A to form acetyl CoA.

fermentation

ATP-generating pathways in which electrons are removed from one organic compound and passed to another organic compound

What process regenerates NAD+ in glycolysis?

formation of ethanol from pyruvate

What does pyruvate carboxylase require?

the coenzyme thiamine pyrophosphate (derived from vitamin thiamine (B1)

alcoholic fermentation

the conversion of glucose into two molecules of ethanol

The NADH generated by glyceraldehyde 3-phosphate dehydrogenase (step 6 in Glycolysis) is oxidized by alcohol dehydrogenase, regenerating NAD+

Describe this diagram.

This how redox balance is maintained in alcoholic fermentation.

• The electron donor is glyceraldehyde 3-phosphate.

• The electron acceptor is acetaldehyde

lactic acid fermentation

the conversion of glucose into two molecules of lactate.

NADH can also be oxidized by converting pyruvate to lactate in a reaction catalyzed by lactate dehydrogenase

Describe this diagram.

This shows that fermentations are a means of oxidizing NADH (Lactic Acid Fermentation)

• The electron donor is glyceraldehyde 3-phosphate.

• The electron acceptor is pyruvate.

Describe this diagram.

This how redox balance is maintained in lactic acid fermentation.

In lactic acid fermentation, lactate dehydrogenase oxidizes NADH to produce lactic acid and regenerate NAD+.

What is some evidence of fermentations providing usable energy in the abscence of oxygen?

Obligate anaerobes cannot survive in the presence of oxygen. Some obligate anaerobic microorganisms are pathogenic.

Many food products, including sour cream, yogurt, various cheeses, beer, wine, and sauerkraut, result from fermentation.

Lactic acid fermentation and alcoholic fermentation are oxidation–reduction reactions. Identify the ultimate electron donor and electron acceptor.

For lactic acid fermentation, the ultimate electron donor is glyceraldehyde 3-phosphate, which is oxidized. The ultimate electron acceptor is pyruvate, which is reduced. Net Oxidation-reduction of the rxn is 0 and the final product is LACTATE.

For alcoholic fermentation, the ultimate electron donor is glyceraldehyde 3-phosphate, which is oxidized and the ultimate electron accepter is acetaldehyde, which is reduced. The final product is ETHANOL.

_______________ from ________________ or high-fructose corn syrup and __________ from ___________________ converted into glycolytic intermediates.

fructose, table sugar, galactose, milk sugar

Describe this diagram.

This shows entry points in glycolysis for galactose and fructose

What is fructose metabolize by in the liver?

fructose 1-phosphate pathway

What is fructose phosphorylated by in adipose tissue?

directly by hexokinase

Describe this diagram.

This shows fructose metabolism.

Fructose enters the glycolytic pathway in the liver through the fructose 1-phosphate pathway.

How is galactose converted into glucose 6-phosphate

by the galactose–glucose interconversion pathway, which begins with the phosphorylation of galactose by galactokinase

Describe this diagram.

This shows the reactions of galactose metabolism.

What does this equation show?

The sum of the reaction of the galactose–glucose interconversion pathway

Which enzyme helps convert glucose 1-phosphate into glucose 6-phosphate?

phosphoglucomutase