Carbohydrate pathways terms

Glycolysis — The 3 Regulatory Steps (enzymes) (ALL irreversible)

(1)Hexokinase / Glucokinase

(3)PFK-1

(10)Pyruvate kinase

Hexokinase / Glucokinase does

Glucose → glucose-6-P

• hexokinase prevents hoarding of too many glucose

  • inhibited by glucose-6-P (post making it)

  • HIGH aff. low Km

PFK-1 is

Major rate-limiting/regulatory step

• acts as valve

• has allosteric activators

  • AMP → HIGH = energy is used up (signals cell is low on energy)

  • fructose-1,6-bisphosphate

• has allosteric inhibitors

  • ATP → signals cell already has plenty of energy

  • Citrate → high citrate = downstream power plants (inter. of TCA cycle) is full..indicating PFK-1 to slow down input

Pyruvate kinase

PEP → pyruvate + ATP

Types of Regulation

Allosteric, Covalent, Induction

Quick Regulation

Allosteric and Covalent

• both sensitive

Long-term regulation

induction

• involuntary regulator

Allosteric

Glycogenesis example

• G6P activates glycogen synthase

  
  

What changes?

• Enzyme activity

  
  

Covalent

Glycogenesis example

• Insulin → dephosphorylation → glycogen synthase ON

  
  

What changes?

• Enzyme activity

  
  

Induction

Glycogenesis example

• Insulin → ↑ glucokinase expression

  
  

What changes?

• Amount of enzyme

Glycogenesis

When? After any meal or in excess food intake

Where is glycogen stored?skeletal muscle, liver

Why? for stored energy

Glycogenolysis

When & why does this occur? in btwn meals, before eating, during exercise, & sleeping (excess energy in blood)

Where does this occur? Liver & skeletal muscle

Glycogenolysis regulation:

Covalent regulation & allosteric regulation

Covalent regulation

• Glucagon / catecholamines → ↑ glycogenolysis

• Insulin → ↓ glycogenolysis

allosteric regulation

• AMP → ↑ glycogen phosphorylase

• ATP, glucose-6-phosphate → ↓

Liver (in Glycogenolysis regulation)

• Less responsive to cellular energy signals

• Glucagon promotes glycogenolysis

• Mobilizes glycogen to help maintain blood glucose

Muscle (in Glycogenolysis regulation)

• More responsive to AMP/ATP and G6P

• Mobilizes glycogen for its own energy needs

You're a skeletal muscle cell during intense exercise. ATP is falling and AMP is rising. What should happen to glycogen phosphorylase?

It gets turned on! Since →

• AMP acts as red alert, signaling energy depleted

If AMP is high in a working muscle, should glycogen phosphorylase be more or less active?

should be more active. WHY?

• high lvls of AMP serve as allosteric activator (signaling low cellular energy) that drives glycogen phosphorylation into its active state to rapidly breakdown storeed glycogen into

  • glucose for energy

TCA Cycle purpose

To completely oxidize Acetyl-CoA (derived from carbohydrates, fats, and amino acids) into carbon dioxide, capturing high-energy electrons in NADH & FADH2 to fuel the Electron Transport Chain (ETC)

Total Energy Yield per Acetyl-CoA (in TCA Cycle)

• 3 NADH

• 1 FADH2

• 1 GTP/ATP

• 2 CO2

Key Regulation of the TCA Cycle

• Isocitrate Dehydrogenase (Step 3):

  • Activators: ADP & Ca²+ (signal low cellular energy and muscle contraction/high demand).

  • Inhibitors: ATP and NADH (signal high cellular energy state).

• alpha-Ketoglutarate Dehydrogenase Complex (Step 4):

  • Activators: Ca²+

  • .Inhibitors: ATP, NADH, and Succinyl-CoA (product inhibition).

• Citrate Synthase (Step 1):

  • Inhibitors: $\text{ATP}$, $\text{NADH}$, Citrate, and Succinyl-CoA (ensures the cell doesn't burn acetyl-CoA when energy reserves are full)

The Pyruvate Dehydrogenase Complex (PDC) rxn

Pyruvate +CoA + NAD^+ → Acetyl-CoA + CO2 + NADH

• will after enter Kreb’s/TCA Cycle

3 enzymatic subunits of PDC

• E1 (Pyruvate Dehydrogenase): Catalyzes the decarboxylation of pyruvate (removes CO2.

  • requires TPP

    • (Note: This is the rate-limiting step of the complex).

• E2 (Dihydrolipoyl Transacetylase): Transfers the remaining acetyl group to Coenzyme A to form Acetyl-CoA.

• E3 (Dihydrolipoyl Dehydrogenase): Regenerates the oxidized form of lipoamide and passes electrons to NAD^+ to produce NADH

The 5 essential cofactors of PDC

• Thiamine Pyrophosphate (TPP)

• Lipoic Acid (Lipoamide) → attached to E2

• Coenzyme A (CoA)

• FAD → used by E3

• NAD^+

Regulation of PDC (Kinase vs Phosphatase)

• PDK

  • Phosphorylates and inactivates the PDC when energy is abundant (high ATP, Acetyl-CoA, and NADH), preventing unnecessary glucose breakdown.

• PDP

  • Dephosphorylates and activates the PDC when energy is low or when calcium Ca²+ surges during muscle contraction.

PDK in regulation of PDC

Phosphorylates and inactivates the PDC when energy is abundant (high ATP, Acetyl-CoA, and NADH), preventing unnecessary glucose breakdown.

PDP in regulation of PDC

Dephosphorylates and activates the PDC when energy is low or when calcium Ca²+ surges during muscle contraction.

Electron Transport Chain (ETC) substrates/inputs

• Electron Donors: Reduced coenzymes NADH and FADH2 (produced from glycolysis, the TCA cycle, and $\beta$-oxidation).

• Terminal Electron Acceptor: Molecular oxygen

• Substrates for Complex V: ADP and inorganic phosphate (Pi)

Electron Transport Chain (ETC) cofactors & Mobile Electron Carriers

• Flavin Mononucleotide (FMN) and Iron-Sulfur (Fe-S) clusters (within complexes).

• Coenzyme Q (CoQ / Ubiquinone):

  • A small, hydrophobic lipid-soluble mobile carrier that shuttles electrons from Complexes I and II to Complex III.

• Cytochrome c (Cyt c):

  • A small, water-soluble mobile protein carrier that shuttles electrons from Complex III to Complex IV.

ETC Complex 1 (NADH-CoQ Reductase)

Role & Proton Pumping →

• Accepts electrons from NADH, passes them to CoQ, and pumps protons

ETC Complex 2 (Succinate-CoQ Reductase)

Role & Proton Pumping →

• Accepts electrons from FADH2 (part of the TCA cycle enzyme succinate dehydrogenase) and passes them to CoQ. Does not pump protons

ETC Complex 3 (CoQ-Cyt c Reductase)

Role & Proton Pumping →

• Passes electrons from CoQ to Cytochrome c and pumps protons

ETC Complex 4 (Cytochrome c Oxidase)

Role & Proton Pumping →

• Passes electrons from Cyt c to molecular oxygen to form water, and pumps protons

ETC Complex 5 (ATP Synthase)

Role & Proton Pumping →

• Uses the proton motive force to synthesize ATP from ADP and Pi

ETC Products.Outputs

• NAD^+ & FAD (oxidized forms, which cycle back to pick up more electrons in pathways like the TCA cycle).

• Water (formed at Complex IV when oxygen accepts the spent electrons and combines with protons).

• An Electrochemical Proton Gradient (H^+ gradient across the inner mitochondrial membrane).

ETC Regulation

By cellular availability of ADP

• Plenty ATP = ADP low = proton gradient backs up, & ETC slows down

• In times of energy demand spikes, ADP floods matrix, driving ATP Synthase & speeds up entire chain

O2 Availability

• low O2 halts complex 4 cuz is ultimate sink for electrons, halts ATP Production

How ATP Synthase Works (The Molecular Turbine)

acts like biological rotar motor. Consists of 2 main functional units →

1. F_0 Domain:

   1. The transmembrane channel through which protons flow down their concentration gradient back into the mitochondrial matrix.

      1. As protons pass through, they physically turn the rotor ring.

2. F_1 Domain:

   1. The catalytic headpiece projecting into the matrix. The rotation of the F_0 subunit induces structural shape changes in the F_$ head,

      1. forcing it to physically bind ADP & Pi together to synthesize ATP

How many protons ($\text{H}^+$) are needed to form 1 ATP?

Approximately 3 to 4 protons must flow through ATP Synthase to generate one molecule of ATP

What happens to ATP after it is created?

ATP is immediately exported out into the cytosol via a transport protein called Adenine Nucleotide Translocase (ANT)

→ (in cytosol)

is rapidly consumed to fuel cellular work (Na^+/K^+ ATPase, muscle contraction)