Glucose Catabolism Flashcards by Elizabeth Quiring

2 Major pathways for the catabolism of glucose

Glycolysis

2. Pentose Phosphate Pathway

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Where does glycolysis occur?

in the cytosol of all cells

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2 stages of glycolysis

Energy investment

2. Energy Recovery

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Reaction 1 of Glycolysis

Glucose

Glucose –> G6P

catalyzed by hexokinase

transfer of a phosphoryl group from ATP to glucose to form G6P

IRREVERSIBLE. Therefore a site of regulation, but not the most important one

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What is a kinase?

an enxyme that transfers phosphoryl groups between ATP and a metabolite

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Can glc transfer Pi to ATP? Why or why not?

No. Because the free energy of the reaction is not favorable. However, ATP can transfer Pi to glc because the free energy change is very negative in both standard and cellular conditions.
delta G for the removal of Pi from ATP is a larger negative number than that for glc-6-P

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Reaction 2 of glycolysis

G6P

G6P F6P

catalyzed by phosphohexose ISOMERASE

Carbonyl group moves from C1 (glc) to C2 (fru)

REVERSIBLE

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Reaction 3 of glycolysis

F6P

F6P –> Fructose 1,6-bisphosphate

catalyzed by phosphofructokinase-1 (PFK-1)

Transfer of Pi from an ATP to a hydroxyl on C1

IRREVERSIBLE. Key site of regulation

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Reaction 4 of glyolysis

F-1,6-bP

F-1,6-bP DHAP and GAP

6 carbon molecule to two 3 carbon molecules

REVERSIBLE

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Reaction 5 of glycolysis

DHAP

DHAP GAP

Catalyzed by triose phosphate ISOMERASE

REVERSIBLE

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Reaction 6 of glycolysis

GAP

GAP + Pi 1,3-Bisphosphoglycerate

Catalyzed by glyceraldehyde 3-phosphate dehydrogenase

Used an NAD+ (2 per glc)

Oxidation of an aldehyde into a carboxylic acid and phosphorylation of the carboxylic acid

REVERSIBLE

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What do dehydrogenases do?

catalyze the redox reactions resulting in the reduction of the co-enzymes NAD+ or FAD

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Reaction 7 of glycolysis

1,3-bPglycerate

-Transfer of a Pi from a carboxylic acid to ADP. Product: 3 phosphoglycerate

Reversible

phosphoglycerate kinase`

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Reaction 8 of glycolysis

3-phosphoglycerate

Reversible

Forms 2-phosphoglycerate in order to create energetically favorable conditions for the removal of phosphate

phosphoglycerate mutase

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Reaction 9 of glycolysis

2-phosphoglycerate

2-phosphoglycerate loses an H2O to form phosphoenolpyruvate.

Enolase

Reversible

This provides sufficient energy to drive the phosphorylation of ADP

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Reaction 10

Phosphoenolpyruvate

transfer of an enol’s phosphate to ADP to form pyruvate + ATP

pyruvate kinase

3rd irreversible step

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Net number of ATP’s and NADH’s produced in glycolysis

2 ATP and 2 NADH

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Why is it useful to create an enol in order to produce pyruvate?

due to the tautomerization of phosphoenol pyruvate and of pyruvate which provides a lot of free energy.
The phosphorylation of ADP into ATP has a positive delta G. The overall net enzyme catalyzed reaction for the production of pyruvate from 2-phosphoenolpyruvate is still neg. and therefore favourable.

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How does the cell regenerate NAD+?

Aerobically (in respiratory chain) or anaerobically (to make lactate or ethanol, depending on conditions)

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Ethanol fermentation

useful in the regeneration of NAD+
Pyruvate –> acetaldehyde –> ethanol
pyruvate decarboxylase; alcohol dehydrogenase

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If methanol were the substrate for alcohol dH, what would form?

Formaldehyde

3 ways to alter enzyme activity

alteration in [enzyme] by altering synthesis and degradation (slow)
Covalent modification of enzymes (faster)
Allosteric control (very fast)

Control of enzyme by covalent modification

other enzymes add or remove groups from the target enzyme, altering its activity

What is the flux control coefficient?

C = 1.0 means total control
C = 0 means no control

Which has a higher Km? hexokinase or glucokinase?

glucokinase has a higher Km

By what is hexokinase inhibited?

hexokinase is inhibited by its product, G6P

What happens if PFK-1 is "off" in the muscle?

its substrate F6P will increase, causing G6P to increase and thus turning off hexokinase.

PFK-1 contains binding sites for:

F6P, ATP, ADP, AMP, and F-2,6-bP

In which state is PFK-1 less active?

less active in T-state, more active in R-state

To what form do INHIBITORS bind best?

to the LESS ACTIVE form

To what form do ACTIVATORS bind best?

to the MORE ACTIVE form, increasing activity

Does ATP bind better to the T or the R state?

ATP binds equally well to both states

What state does F6P bind to best on PFK-1?

F6P binds strongly to the R state

To what state does ATP bind to best in the allosteric binding site in PFK-1?

ATP binds more strongly to the T-state | ADP binds to the R-state

What effect does the low concentration of ATP have on the flow of glycolysis

low ATP = cell activity increases, making flow through glycolysis increase

Is AMP an activator or deactivator in PFK-1?

AMP is a strong activator and binds to the R state

Explain what a small change in [ATP] does to [AMP]

a small change in [ATP] leads to a very large change in [AMP].

What inhibits pyruvate kinase in the cell?

high concentrations of ATP. Aka when the cell has a high energy status

Galactose and Mannose differ from glucose in which carbon?

Gal: C-4 Man: C-2

What does the phospholysis of glycogen produce?

glc-1-P which goes to glc-6-P by phosphoglucomutase

What does lactase hydrolize lactose into?

gal and glu

What does sucrase hydrolize sucrose into?

glu and fru

Where does Pentose Phosphate Pathway take place?

in the cytosol of all cells

2 branches of PPP

oxidative (not reversible) | non-oxidative (reversible)

Oxidative branch step 1 (starting with G6P)

converts G6P into 6-phosphoglucono-lactone by G6P dehydrogenase site of regulation (irreversible) produces and NADPH Regulated by [NADPH]

what effect on the speed of oxidative branch does a high concentration of NADPH have?

pathway slows down with high [NADPH]

Oxidative branch step 2 (starting with 6-phosphoglucono-lactone)

forms 6-phosphogluconate by lactonase and the addition of H2O ketone on C1 to carboxylate group

Oxidative branch step 3 (staring with 6-phosphogluconate)

loss of C1 in the form of CO2 and production of ribulose-5-phosphate by a dehydrogenase enzyme imput of 1 NADP+ output of NADPH and CO2

ribulose-5-phosphate is convertedinto _______ by phosphopentose isomerase

ribose-5-phosphate

How many glc-6-P go into PPP oxidative branch and yield how many CO2's and rib-5-P's?

6 G6P (36 C's) yield 6 CO2 (6C's) and 6 rib-5-P (30C)

Non-oxidative branch of PPP converts the 6 rib-5-P into what?

5 glc-6-P molecules