302: Transporters and Glycolysis

Metabolic Pathway

Begins with a specific molecule, ends with specific product connected by intermediates only flowing in one direction

Essentially irreversible

Only flows in one direction, often regulated

Reversible

Direction depends on relative concentrations

Chemical Energy

ATP and high energy phosphate bonds

Electrical energy

NADH (reducing equivalents), can be converted into ATP (chemical)

Glucose

• 6 carbon monosaccharide

• Most important fuel (especially for brain)

• Stored in liver, skeletal muscles

• Synthesized by liver and kidney cortex

How is ATP derived?

Oxidation of glucose (glycolysis and ox. metabolism)

4 major glucose metabolism pathways

1. Glycolysis

2. Gluconeogenesis

3. Glycogenesis

4. Glycogenolysis

Pathways performed by liver

ALL: glycolysis, gluconeo., glycogenesis, glycogenolysis

Pathways performed by skeleted muscle:

Glycolysis, glycogenesis, glycogenolysis

Pathways performed by brain/adipose:

Glycolysis only

‘lysis’ means…

Oxidation, production of ATP (substrate level), catabolism (dehydrogenase)

‘genesis’ means…

Reduction, breakdown of ATP, anabolism (reductase)

Important GLUT transporters

GLUT 2 and 4 (transport glucose to/from cells)

GLUT 2

Bidirectional transporter that transports glucose based on concentration gradients in liver and blood

High glucose (after eating)

GLUT 2 transports glucose into liver

GLUT 4 expression, transports glucose to skeletal muscle

Low glucose (fasting)

GLUT 2 transports blood OUT of liver into blood

GLUT 4 has low/no activity

GLUT 4

Unidirectional transporter expressed on skeletal muscle cell surface only when high glucose due to insulin signaling

Signaling of GLUR 4 

Insulin-dependent (high blood glucose = insulin = expression)

Affinity of GLUT 2

Relatively low

Affinity of GLUT 4

Relatively high

Glycolysis

CENTRAL metabolic pathway to generate energy/intermediates for other pathways

O₂ requirement of Glycolysis

NONE, only need glucose and ATP!

Main function of Glycolysis

Energy generation (ATP and NADH), limited if anaerobic

What type of pathway is glycolysis?

Catabolic: glucose is cleaved and oxidized to pyruvate

Steps of Glycolysis:

10 total; 5 investment, 5 generation

Glycolysis Location

Cytoplasm

Net gain of ATP and NADH in glycolysis

2 and 2

Technically 4 ATP, but 2 is used in investment phase

1st Step of Glycolysis

Glucose + ATP → Glucose-6-phosphate (G-6-P)

IRREVERSIBLE coupling

Regulated

USES ATP (gives phosphate)

Hexokinase/Glucokinase

Step 1 enzymes that attach phosphate to Glucose from ATP

Which enzyme traps glucose in cell?

Hexokinase or Glucokinase (GK in liver)

Hexokinase

High-affinity step 1 cytoplasmic enzyme that is easily saturated, broad specifity

Glucokinase

Low-affinity step 1 enzyme found in liver that allows for synthesis of glucose, other hexoses are NOT substrates

Step 2

G-6-P → F-6-P via isomerase

Reversible rearrangement of atoms

Step 3

F-6-P + ATP → F-1,6-BP

IRREVERSIBLE, primary site of carbon flow regulation

RATE LIMITING

Phosphofructokinase-1

Phosphofructokinase-1

MOST regulated enzyme of glycolysis

Attaches 2nd phosphate to F-6-P

Step 4

F-1,6-BP cleaved in half, reversible by aldolase

Step 5

Generates 2 Glyceraldehyde-3-Phosphate with isomerase

Reversible

Phase 2: Generation

Runs twice per glucose, generating 4 ATP, 2 NADH

Step 6

Glyceraldehyde-3-phosphate Dehydrogenase

1st investment step (reversible)

GENERATES NADH using phosphate

Step 7

Phosphoglycerate Kinase
GENERATES ATP - 2 molecules (substrate-level phosphorylation)

Reversible

Step 8

Phosphoglycerate mutase (rearrangement)

Reversible

Step 9

Enolase

Dehydration reaction forming phosphoenolpyruvate

Reversible

Step 10

Pyruvate Kinase

GENERATES ATP - 2 molecules (substrate level)

IRREVERSIBLE, regulated

Pyruvate Kinase

Phosphorylates ADP to ATP, by transferring phosphate from PEP

All end products of glycolysis

1. 2 ATP net

2. 2 NADH

3. 2 Pyruvate

Important Enzymes:

1. HK (high affinity) - phosphorylate glucose (use 1 ATP, coupling)

2. GK (low affinity) - phosphorylate glucose (use 1 ATP, coupling)

3. Phosphofructokinase-1 - MAIN regulation of carbon flow, adds second ATP

4. Glyceraldehyde-3-phosphate dehydrogenase - generates NADH

5. Phosphoglycerate kinase - generates ATP

6. Pyruvate kinase - generates ATP, produces pyruvate