2 - Metabolic Pathways (Biochemistry II)

Yeast fermentation:

pyruvate turns into ethanol (NADH -> NAD+)

Glycolysis location:

cytoplasm

Which process requires O2?
I. glycolysis
II. PDC
III. Krebs cycle
IV. ETC/oxidative phosphorylation

II (indirectly)
III
IV

Glycolysis:

turning a glucose (6C) to 2 pyruvates (3C x2)

Hexokinase:

key enzyme in glycolysis that turns glc into glucose-6-phosphate (G6P) with 1 ATP -> ADP + P. Once this occurs, glycolysis process is COMMITTED.

What molecule(s) REGULATE hexokinase?

glucose-6-phosphate (G6P)

Why is hexokinase phosphorylation step considered as glc being committed to glycolysis?

Phosphorylation rxn is VERY IRREVERSIBLE (delta G <<< 0)

Phosphofructokinase (PFK):

After G6P -> F6P, PFK phosphorylates it AGAIN -> P-6C-P, with ATP -> ADP + P

What molecule(s) REGULATE PFK?

- ATP (is PFK's substrate AND inhibitor)
- citrate

Where does substrate ATP bind to in PFK?

its active site (has HIGHER affinity for ATP than allosteric site)

Where does inhibitor ATP bind to in PFK?

its allosteric site

Why does PFK active site have a higher affinity for ATP than its allosteric site?

so that glycolysis would continue, especially in conditions where ATP is low. We only want to halt glycolysis when there is ATP is ABUNDANT

Pyruvate kinase:

DEPHOSPHORYLATES (not phosphorylates) 2-3C-P to make pyruvate

What molecule REGULATES pyruvate kinase?

acetyl CoA

PDC:

convert pyruvate (3C) into acetyl CoA (2C)

Products of PDC:

- 1 CO2
- 1 NADH
(technically x2 bc there are 2 pyruvate molecules)

PDC location:

Mitochondria matrix

oxidative decarboxylation:

removing 1 carbon off of pyruvate to make CO2 byproduct.
- Pyruvate is oxidized
- NAD+ is reduced to NADH

Krebs cycle:

combines acetyl CoA (2C) with oxaloacetate (4C) to ultimately make a LOT of NADH, FADH2, and CO2

citrate (tricarboxylic acid):

6C molecule made from combination of oxaloacetate and acetyl CoA

Steps of Krebs cycle:

- oxaloacetate (4C) + acetyl CoA (2C) = citrate (6C)
- citrate (6C) -> beta-ketoglutarate (5C) (CO2 and NADH)
- beta-ketoglutarate (5C) -> succinate (4C) (CO2 and NADH)
- succinate (4C) -> fumarate (GTP)
- fumarate -> malate (FADH2)
- malate -> oxaloacetate (NADH)

Total # of NADH molecules made during Krebs cycle:

3
- 1 from citrate -> beta-ketoglutarate
- 1 from beta-keto -> succinate
- 1 from malate -> oxaloacetate

Total # of CO2 molecules made during Krebs cycle:

2
- 1 from citrate -> beta-keto
- 1 from beta-keto -> succinate

Total # of GTP molecules made during Krebs cycle:

1, from succinate -> succinate

Total # of FADH2 molecules made during Krebs cycle:

1, from fumarate -> malate

What are Krebs cycle's byproducts?

- 3 NADH
- 2 CO2
- 1 GTP
- 1 FADH2
(technically x2 for BOTH pyruvates)

Krebs cycle location:

mitochondria matrix

ETC/oxidative phosphorylation location:

inner mitochondrial membrane

ETC/oxidative phosphorylation goals:

- oxidize e- carriers (empty out NADH and FADH2)
- make a LOT of ATP

Why does NADH generate more energy than FADH2, even though FADH2 has 2e- to oxidize?

- NADH drops of 2- at the FIRST e- acceptor complex (complex I) so it can move more H+ across.
- FADH2 drops off e- at coenzyme Q (CoQ) and that is after complex I

What happens if O2 is not available to accept e-?

pyruvate then accepts electrons and begins fermentation

How many ATP molecules does 1 NADH generate?

1 NADH = 2.5 ATP

How many ATP molecules does 1 FADH2 generate?

1 FADH2 = 1.5 ATP

How do NADH made from glycolysis get to ETC?

NADH oxidize themselves and e- move from cytosol (glycolysis) to COENZYME Q (w/ FADH2's e-)

What happens to Krebs cycle and PDC when there is no O2?

NADH and FADH2 can't empty their electrons w/o O2, so both are indirectly independent on O2

What happens in anaerobic conditions?

- No ETC/oxidative phosphorylation
- No Krebs cycle
- No PDC

What are the products at the end of glycolysis?

2 ATP, 2 NADH, and 2 pyruvate

Lactic acid fermentation (for muscles, etc.):

pyruvate turns into lactic acid (NADH -> NAD+)

Problems with fermentation:

- end products are toxic (yeast, lactic acid) bc pH decrease -> muscle proteins disrupted
- not enough ATP (2 ATP/glucose)

Conditions that activate glycolysis:

- high [glc]
- low ATP

Conditions that activate gluconeogenesis:

(opposite of glycolysis)
- [glc] low
- ATP high

Gluconeogenesis first steps:

- 2 pyruvates turned into 2 oxaloacetate (4C) via pyruvate carboxylase (needs TWO ATP)
- 2 OAA turned into 2 PEP (3C) via PEPCK (needs 2 GTP)

Which processes are reciprocal rxns?
I. Gluconeogenesis
II. ETC
III. Glycolysis

I and III

When does gluconeogenesis occur?

When fats, AA, prots, etc. are used in cellular respiration (bc glucose is low), gluconeogenesis occur.

this is bc BRAIN NEEDS GLUCOSE!!!

Reciprocal regulation:

The SAME molecule regulates 2 opposing enzymes in opposite ways

Hormonal regulation:

when hormones regulate enzymes (ex: insulin increase F-2,6-BP, which increase PFK and glycolysis

When does glycogenesis occur?

when blood glucose is HIGH, to make glycogen from glc -> glycogen stored in liver and a little in skeletal muscles

When does glycogenolysis occur?

breakdown of glycogen to make glucose; produces epinephrine and glucagon

Phosphoglucomutase:

Enzyme that mutates the placement of phosphate on glucose carbon (1st step in glycogenesis from glycolysis)

What can phosphoglucomutase do? (2 things)

- change the placement of phosphate on G-6P to G1P to kickstart glycogenesis
- perform the reverse rxn back to glycolysis

Steps in glycogenesis:

1. Phosphoglucomutase changing phosphate placement at G6P to G1P
2. Glycogen synthase turns G1P to glycogen

Steps in reverse rxn of glycogenesis:

1. Glycogen phosphorylase turns glycogen to G1P
2. Phosphoglucomutase turns G1P to G6P

What molecule stimulates glycogen phosphorylase?

Glucagon

Pentose phosphate pathway (PPP):

used to make precursors for other energy producing pathways, but it is NOT an energy producing pathway.

Reactants of PPP:

G6P (from glycolysis)

Products of PPP:

Ru5P (nucleotide synthesis), GAP and F6P (goes to glycolysis)

Purpose of PPP:

- NADP+ reduction to NADPH to neutralize ROS
- Ru5P (building block for nucleotide synthesis)

Which phase of PPP is irreversible? Why?

oxidative phase (initial) b/c delta G <<< 0

Where does fatty acid catabolism (oxidation) occur?

mitochondria

Product of fatty acid oxidation:

acetyl CoA

Location of fatty acid synthesis:

cytoplasm

Starting materials of fatty acid synthesis:

Acetyl CoA (2C) and malonyl CoA (3C), carried by fatty acid synthase (FAS)

Does FA oxidation generate/expend a lot of ATP?

GENERATES a lot of ATP and electron carriers (NADP+ -> NADPH)

Does FA synthesis generate/expend a lot of ATP?

NEEDS a lot of ATP (and electron carriers NADPH -> NADP+)

When is ketogenesis activated?

During long-term starvation and BGL falls

What happens during ketogenesis?

fatty acids are oxidized to make acetyl CoA --> go into Krebs OR acetyl CoAs will react together to form KETONE BODIES

Ketone bodies:

can enter brain and be reconverted into acetyl CoA -> primary nrg for brain during starvation

Why is ketoacidosis in diabetics critical?

Ketone bodies are ACIDS and travel easily in blood, but it lowers blood pH

Conditions that activate glycogenesis:

high insulin so that glycogen can get stored

Conditions that activate glycogenolysis:

high conc. of GLUCAGON, EPINEPHRINE

Conditions that inhibit glycogenesis:

glucagon

Conditions that activate Krebs cycle:

high NAD+ and FAD substrates, ADP

Conditions that inhibit Krebs cycle:

high conc. NADH, ATP

Conditions that inhibit gluconeogenesis:

insulin

Conditions that inhibit fatty acid synthesis:

glucagon and epinephrine

Conditions that stimulates fatty acid synthesis:

insulin

Conditions that activates ketogenesis:

glucagon, cortisol, thyroid hormones

What is the PRIMARY regulator of ketogenesis? What does it do to the process?

Insulin, inhibits ketogenesis.