Protein Catabolism + Uncoupling Agents

Protein roles

Not mainly used as fuel, their primary roles are structural and functional (muscles, enzymes, hormones)

Only about 10% of the body’s ATP normally comes from protein

Protein use for energy increases during fasting, when glucose and glycogen are low

Dietary Proteins

Dietary proteins are broken into amino acids during digestion

These amino acids travel through blood

They are used to build and repair body proteins

Extra amino acids can also be used to produce energy

Amino Acids

When amino acids are broken down, they produce ammonia (NH3), which is toxic and must be converted to urea (urea cycle done in liver)

Amino acids can also be turned into energy-producing molecules (at least indirectly) e.g. glucose, fatty acids, acetyl CoA

Amino Acid Breakdown Pathway

Transamination: the amino acid transfers its nitrogen to form glutamate

Deamination: glutamate loses the nitrogen, producing ammonia

Urea cycle: ammonia is converted to urea, which is safely excreted

Amino acids can be glucogenic/ketogenic

The carbon skeleton of the amino acid can be converted into:

Glucose

Fatty acids

Acetyl CoA

The amino group is not useful in producing energy

Uncouplers

Decouple the proton gradient from ATP synthesis, reduce or stop proton flow through ATP Synthase

Normally: ETC pumps H+ into the intermembrane space. H+ flows through ATP Synthase → makes ATP

With uncouplers:

ETC still pumps H+ out normally

But H+ leaks back into the matrix due to the uncoupler (NOT through ATP Synthase)

ATP Synthase can’t work properly → much less ATP is made.

The proton gradient energy is released as heat

Effects of Uncouplers

ETC continues working: because ETC is not blocked, electrons keep flowing and O2 is still consumed.

Protons leak back into matrix: this destroys the proton gradient needed to power ATP synthase

ATP production decreases: ATP Synthase is bypassed

Heat increase: all the energy that would make ATP is lost as thermal energy

Oxygen consumption increases, because ETC speeds up trying to rebuild the lost gradient

Examples of Uncouplers

DNP (2,4- dinitrophenol) synthetic, dangerous weight-loss chemical

Thermogenin (UCP1) natural uncoupler in brown fat used for heat generation

UCP2: similar uncoupling protein found in many tissues

Thyroxine (T4) high levels can act as a mild uncoupler

Respiratory Chain Inhibitors

Also impact ETC, but the mechanism by which they work is different. Here the ETC actually stops working entirely, a common example is cyanide.

Cyanide binds to complex IV

Blocks O2 from being reduced to H2O

Electron flow stops completely

ATP production stops

Oxygen consumption falls to zero