Protein Metabolism Overview
📌 Protein digestion breaks down proteins into amino acids absorbed in the intestines and transported to the liver for metabolism.
📉 Amino acid catabolism involves transamination and deamination, producing ammonia ($\text{NH}_4^+$) and carbon skeletons.
🔄 Ammonia is typically converted into urea via the urea cycle for excretion in urine, while carbon skeletons enter the citric acid cycle for energy production.

Amino Acid Catabolism: Deamination and Urea Cycle
🔬 Transamination transfers the amino group ($\text{NH}_2$) from an amino acid to alpha-ketoglutarate, forming glutamate.
🔗 Deamination of glutamate, catalyzed by glutamate dehydrogenase in the mitochondrial matrix, releases $\text{NH}_4^+$.
💀 Ammonia ($\text{NH}_4^+$) must be detoxified because it is poisonous to the body; vertebrates excrete it as urea.
🐠 In contrast, fish excrete ammonia directly, and birds (aves) excrete it as uric acid.

Carbon Skeleton Fate and Anabolism
➡️ Carbon skeletons enter the citric acid cycle via various intermediate compounds, depending on the original amino acid structure.
🌱 Non-essential amino acids can be synthesized in animals from intermediate compounds in glycolysis (e.g., 3-phosphoglycerate) or the citric acid cycle (e.g., oxaloacetate, $\alpha$-ketoglutarate).
🔗 Amino acids like leucine, lysine, phenylalanine, tryptophan, and tyrosine must first be converted to acetoacetyl-CoA before entering the citric acid cycle.

Protein Biosynthesis (Translation)
⚙️ Protein biosynthesis involves two main stages: transcription (DNA to RNA) and translation (mRNA to amino acid sequence).
🧬 Key components in translation include the ribosome (made of rRNA and protein), mRNA (carrying the code), tRNA (carrying amino acids), and the 20 types of amino acids.
🔄 Elongation involves sequential addition of amino acids based on mRNA codons, utilizing tRNA binding sites (A, P, and E sites) within the ribosome.
🛑 Translation terminates when a release factor binds to a stop codon on the mRNA, leading to ribosome disassembly.

Post-Translational Processing
🌀 After synthesis, proteins require post-translational processing, most critically, folding into the correct three-dimensional structure for functionality.
⚠️ Misfolded proteins, such as prions, can lead to severe issues, exemplified by diseases like mad cow disease.

Key Points & Insights
➡️ Amino acid catabolism is channeled through the urea cycle for ammonia disposal and the citric acid cycle for energy extraction from carbon skeletons.
➡️ Glutamate is the central molecule for amino group transfer, as it is the only amino acid that readily undergoes deamination to produce $\text{NH}_4^+$.
➡️ Understanding the five stages of translation (activation, initiation, elongation, termination, post-translational modification) is crucial for grasping protein synthesis.
➡️ Essential amino acids must be obtained from the diet, while non-essential amino acids are synthesized from metabolic intermediates like those found in glycolysis or the TCA cycle.

📸 Video summarized with SummaryTube.com on Nov 06, 2025, 22:08 UTC