In nature, amino acids exist as two distinct isomers, designated D and L Isomers. These mirror images of one another are metabolized differently in the cell. Only L isomers are used in cellular protein synthesis. Most catabolic enzymes metabolize L-isomers only while D-isomers are metabolized by D-Amino acid oxidase.

This YouTube video transcript from "Metabolism Made Easy" highlights the significant role of fat reserves in the human body. The speaker emphasizes the substantial quantity of fat, primarily in the form of triacylglycerols (TAGs), stored within us. This reserve represents a considerable percentage of body mass and, crucially, an enormous energy depot. The transcript points out that the caloric potential of fat far surpasses that of both protein and glycogen, making it the body's most important long-term energy source.

In addition to providing significant energy through the oxidation of Acetyl CoA, the TCA cycle plays important roles in anabolic processes like gluconeogenesis, ketogenesis, fatty acid, and cholesterol biosynthesis by providing essential precursors for those processes.

Catabolism of amino acids involves the removal of nitrogen by either specific transaminases or by glutamate dehydrogenase. These pathways will produce alpha-keto acids/ carbon skeletons which can be used for energy production or biosynthesis of other molecules.

Amino acids are defined by two functional groups: An amino group and a carboxylic group. Both groups can donate/accept protons under specific pH conditions. At a neural pH (7.0) amino acids exist in the zwitterionic form of NH3 + and COO-.

The source, an excerpt from the YouTube video "Catabolism of Amino Acids @Metabolism Made Easy," discusses the unique aspects of amino acid metabolism. It explains that amino acids are the sole nitrogen-containing molecules utilized by the body, which leads to the eventual production of ammonia during catabolism. To manage this toxic byproduct, the body employs the urea cycle to safely eliminate the nitrogen. The video also highlights that unlike glucose or fatty acids, the body lacks a storage mechanism for excess amino acids, meaning any surplus not used for synthesizing proteins or specialized products is broken down. This catabolism generates a carbon skeleton or keto acid that can then be used by the body for energy production.

Insulin will activate fatty acid synthesis, triacylglycerol synthesis and cholesterol synthesis by dephosphorylating two key enzymes: acetyl CoA carboxylase and HMG CoA reductase. Insulin will upregulate lipoprotein lipase, increasing uptake of fatty acids from circulating chylomicrons into various tissues. Glucose will provide both precursors for triacylglycerol synthesis and fatty acid biosynthesis.

The video transcript from the "Metabolism Made Easy" YouTube channel focuses on the biological role and derivation of ketone bodies. Specifically, it identifies acetoacetate and beta-hydroxybutyrate as key ketone bodies released by the liver. These compounds are presented as an alternative energy source for various tissues, including the brain, muscles, and other peripheral tissues, particularly during periods of fasting. Finally, the source explains that ketone bodies originate from acetyl CoA, which is itself a product of the beta-oxidation of fatty acids within the liver.