BIOCHEM CH11

Biochemistry II Lecture 11 Notes

Fatty Acid Oxidation Regulation

  • Fatty Acid Oxidation is tightly regulated.

    • Malonyl-CoA is the first intermediate in cytosolic fatty acid synthesis.

      • Malonyl-CoA acts to block the entry of fatty acids into the mitochondria, thereby preventing futile cycling.

      • This regulatory mechanism is facilitated by Acetyl CoA Carboxylase (ACC).

  • High NADH/NAD⁺ ratio

    • Inhibits β-hydroxyacyl-CoA dehydrogenase.

  • Acetyl-CoA feedback regulation

    • High acetyl-CoA concentration inhibits thiolase.

β-Oxidation Pathway Overview

  • A simplified overview of β-oxidation process is depicted as follows:

    • Palmitoyl-CoA (C16) undergoes multiple enzymatic transformations:

    • Initial conversion by FAD via acyl-CoA dehydrogenase, leading to the formation of FADH₂ and trans-Δ²-Enoyl-CoA.

    • Hydrolysis by enoyl-CoA hydratase, creating β-hydroxyacyl-CoA.

    • Dehydrogenation again catalyzed by NAD⁺ yielding β-Ketoacyl-CoA.

    • Splitting by thiolase results in the release of an Acetyl-CoA molecule and a new Acyl-CoA that undergoes further oxidative cycles.

Peroxisomal β-Oxidation

  • Peroxisomes are organelles in both plant and animal cells, which also conduct β-oxidation.

  • The β-oxidation process in peroxisomes has four steps:

    1. Dehydrogenation

    2. Addition of water to the resulting double bond

    3. Oxidation of β-hydroxyacyl-CoA to form a ketone

    4. Thiolytic cleavage by coenzyme A

Amino Acid Oxidation and Urea Production

  • Overview of amino acid catabolism

    • All paths for amino acid degradation are divided into two broad parts concerning:

    • Amino groups

    • Carbon skeletons

    • Pyridoxal phosphate is the crucial cofactor in processes where the α-amino group separates from the carbon skeleton, being redirected to amino group metabolism pathways.

    • The carbon skeletons are metabolized to citric acid cycle intermediates.

Free Ammonia Toxicity and Urea Cycle

  • Free ammonia is toxic and needs to be safely excreted. In mammals, this occurs through the urea cycle.

    • The metabolic overview of amino acid catabolism involves:

    • Biosynthesis of amino acids, nucleotides, and biological amines

    • The utilization of carbamoyl phosphate

    • Pathways involving intracellular proteins

    • Processing dietary proteins to produce amino acids, which yield nitrogen for excretion through the urea cycle

    • The cycle processes components to CO₂, H₂O, and ATP, producing Urea as the primary nitrogen excretion product, with intermediates including oxaloacetate and citric acid cycle involvement

Conditions for Amino Acid Oxidation

  • Under specific metabolic circumstances, amino acids undergo oxidative degradation:

    • During protein turnover when amino acids are not required for new protein production

    • When excessive amino acids are ingested beyond the body's protein synthesis needs

    • When cellular proteins are metabolized for energy due to unavailability or improper utilization of carbohydrates

Pathways of Amino Group Catabolism

  • Significant amino acids, including alanine, glutamate, glutamine, and aspartate, manage the transport and distribution of amino groups.

  • These amino acids can convert readily into key citric acid cycle intermediates:

    • Glutamate to α-ketoglutarate

    • Glutamine to α-ketoglutarate

    • Alanine to pyruvate

    • Aspartate to oxaloacetate

Role of Pyridoxal Phosphate (PLP)

  • Pyridoxal phosphate (PLP)

    • The coenzyme form of vitamin B6, used as a prosthetic group by all aminotransferases.

    • Functions in amino group transport and is covalently linked to the enzyme, specifically to the ε-amino group of a lysine residue.

    • The linkage allows PLP to accept an amino group to form pyridoxamine phosphate, which then can donate the group to an α-keto acid.

Mechanism of Aminotransferases

  • Aminotransferases (transaminases) facilitate the removal of α-amino groups with specificity for various L-amino acids, frequently using α-ketoglutarate as an acceptor.

  • Transamination reactions convert amino acids into α-keto acids while transferring amino groups, yielding α-ketoglutarate as a product.

Overall Transamination Process

  • The PLP mechanism begins with the external aldimine, relocation of the amino group occurs resulting in the formation of an intermediate structure, which eventually yields the corresponding α-keto acid.

L-Glutamate Dehydrogenase

  • This enzyme catalyzes the oxidative deamination of glutamate, generating ammonium ion (NH₄⁺) and α-ketoglutarate.

  • This process occurs in the mitochondrial matrix and can utilize NAD⁺ or NADP⁺.

    • α-Ketoglutarate can proceed into the citric acid cycle or be utilized in gluconeogenesis.

Glutamine Metabolism

  • Glutamine transport plays a vital role, transporting ammonia in the bloodstream. - Glutamine synthetase combines ammonia with glutamate to produce glutamine, a reaction requiring ATP.

    • Subsequently, glutaminase converts glutamine back to glutamate and NH₄⁺ as needed.

Alanine in Nitrogen Transport

  • Alanine also transport ammonia from skeletal muscles to the liver.

    • The alanine aminotransferase mediates the interconversion of pyruvate and alanine through transamination with glutamate.

Glucose-Alanine Cycle

  • This cycle describes how alanine carries ammonia and carbon skeletons from pyruvate to the liver for ammonia excretion and conversion to glucose, which is then sent back to muscle tissue.

Urea Cycle Overview

  • The urea cycle is crucial for converting ammonia, sequestered in the mitochondria of hepatocytes, to urea, which is then excreted in urine.

  • Five enzymatic steps are involved in producing urea:

    1. Carbamoyl phosphate synthetase I: catalyzes the formation of carbamoyl phosphate from NH₄⁺ and CO₂ (as bicarbonate), requiring 2 ATP.

    2. First nitrogen entry occurs from ammonia, with the reaction exhibiting two activation steps involving ATP.