Fate of the Skeletons of Amino Acids

Ketogenic and Glucogenic Amino Acids

These notes are based on the uploaded amino acid metabolism lecture PDFs.

Overview

After removal of the amino group by:

• transamination

• deamination

the remaining:

carbon skeleton (α-keto acid)

enters metabolic pathways for:

• energy production

• gluconeogenesis

• ketogenesis

• lipid synthesis

General reactions in amino acid metabolism include:

• transamination

• deamination

• decarboxylation

• C1 transfer

• oxygenation reactions

• β-oxidation-like pathways

Fate of Amino Acid Carbon Skeletons

The carbon skeletons are converted into:

Glucogenic Amino Acids

Definition

Amino acids whose degradation yields:

• pyruvate
  or

• TCA cycle intermediates

These can be converted into glucose through:

gluconeogenesis

Ketogenic Amino Acids

Definition

Amino acids degraded to:

• acetyl-CoA
  or

• acetoacetyl-CoA

These:

• cannot produce net glucose

• are used for ketone body formation or lipid synthesis

Why Acetyl-CoA Cannot Produce Glucose

Acetyl-CoA enters the TCA cycle, but:

• its carbons are lost as CO₂

• there is no net oxaloacetate production

Therefore:

acetyl-CoA is not gluconeogenic.

Classification of Amino Acids

Purely Ketogenic Amino Acids

Mnemonic:

“LL = purely ketogenic”

• Leucine

• Lysine

Both Ketogenic and Glucogenic Amino Acids

Mnemonic:

“PITT”

• Phenylalanine

• Isoleucine

• Threonine

• Tryptophan
  (+ Tyrosine)

Purely Glucogenic Amino Acids

To Pyruvate

To Oxaloacetate

To α-Ketoglutarate

Mnemonic:

“Glu Gln Pro His Arg → α-KG”

To Succinyl-CoA

Mnemonic:

“VOMIT”

• Valine

• Odd-chain fatty acids

• Methionine

• Isoleucine

• Threonine

(all produce propionyl-CoA → succinyl-CoA)

To Fumarate

Important Metabolic Pathways

Alanine → Pyruvate

Via transamination:

Alanine + \alpha\text{-ketoglutarate} \rightleftharpoons Pyruvate + Glutamate

ALT catalyzes this reaction.

Glutamate → α-Ketoglutarate

Via oxidative deamination:

Glutamate + NAD(P)^+ \rightleftharpoons \alpha\text{-ketoglutarate} + NH_3 + NAD(P)H

Catalyzed by glutamate dehydrogenase (GDH).

Relationship Between Amino Acid Metabolism and Other Pathways

Amino acid degradation is integrated with:

• glycolysis

• TCA cycle

• gluconeogenesis

• fatty acid metabolism

• ketogenesis

Clinical Importance

Hyperammonemia

Occurs when nitrogen elimination is impaired.

Ammonia toxicity causes:

• α-ketoglutarate depletion

• TCA inhibition

• ATP depletion

• glutamate excitotoxicity

• astrocyte swelling from glutamine accumulation

Maple Syrup Urine Disease (MSUD)

Defect in:

• branched-chain α-ketoacid dehydrogenase

Affects:

• leucine

• isoleucine

• valine degradation

Phenylketonuria (PKU)

Defect in:

• phenylalanine metabolism

Phenylalanine and tyrosine normally yield:

• fumarate

• acetoacetate

High-Yield Summary Table

Important Exam Mnemonics

Purely Ketogenic

“LL”

• Leucine

• Lysine

Both Ketogenic and Glucogenic

“PITTT”

• Phenylalanine

• Isoleucine

• Threonine

• Tryptophan

• Tyrosine

Succinyl-CoA Producers

“VOMIT”

• Valine

• Odd-chain FA

• Methionine

• Isoleucine

• Threonine

Key Concept

Amino acids are classified according to:

the metabolic fate of their carbon skeletons after nitrogen removal.