Introduction to Biochemistry - Metabolism - Anabolic, Catabolic - Insulin, Glucagon - Amino Acids

The video introduces biochemistry as the chemistry of life, focusing on metabolism, which involves anabolism (building up, requiring energy) and catabolism (breaking down, releasing energy). Enzymes act as catalysts to speed up these reactions. The body processes carbohydrates, proteins, and fats, converting large molecules into smaller ones through digestion. The pancreas plays a crucial role with its digestive enzymes (amylases, peptidases, lipases).

Metabolism is driven by hormones. Insulin promotes anabolism (building up) in the fed state, facilitating proteogenesis, glycogenesis, glycolysis, and lipogenesis. Glucagon (along with epinephrine, cortisol, and thyroxine) promotes catabolism (breaking down) in the fasting state, leading to proteolysis, gluconeogenesis, glycogenolysis, and lipolysis, which can result in the formation of ketone bodies. Insulin is anti-ketogenic, while a lack of insulin can lead to diabetic ketoacidosis.

Acetyl-CoA is the central metabolic hub, as all major food groups (carbohydrates, proteins, fats, and ketone bodies) are converted into it. Acetyl-CoA then enters the TCA cycle and electron transport chain in the mitochondria to produce ATP (energy). The video also outlines the caloric content of carbohydrates, proteins, fats, and alcohol, emphasizing fat as the most energy-dense storage form.

Amino acids consist of an alpha carbon, an amino group, a carboxyl group, a hydrogen, and an R group, which determines their properties. They are amphoteric due to their basic amino and acidic carboxyl groups. The video highlights the D-sugar and L-amino acid conventions in the human body, with glycine being an exception as it is achiral. Cysteine is also noted for its unique L yet R configuration.

The video clarifies that while there are many amino acids, only 20 are proteogenic, meaning they are incorporated into proteins and are coded for by DNA codons. Non-proteogenic amino acids, like ornithine and homocysteine, are not directly incorporated into proteins. A list of the 20 proteogenic amino acids with their three-letter and one-letter abbreviations is provided.

Amino acids are classified based on their metabolic fate: glucogenic (can form glucose), ketogenic (can form ketone bodies), or both. Lysine and leucine are purely ketogenic, while most amino acids are glucogenic. The video also distinguishes between essential (must be consumed in diet), non-essential (body can synthesize), and semi-essential/conditionally essential amino acids (become essential under specific conditions, like tyrosine in phenylketonuria).

Amino acids are categorized into five groups based on their side chain properties: nonpolar non-aromatic, aromatic, polar non-aromatic, negatively charged (acidic), and positively charged (basic). Examples and key characteristics are given for each group. This includes sulfur-containing amino acids (methionine, cysteine), hydroxy-containing (serine, threonine), and amide-containing (asparagine, glutamine).

The video delves into specific amino acids, detailing their structures and three key facts. Glycine is the smallest, simplest, and achiral amino acid, important for collagen. Alanine converts to pyruvate via ALT, crucial for the Cahill cycle. Methionine contains sulfur, is the start codon, and a methyl group donor (SAM). Branched-chain amino acids (valine, leucine, isoleucine) are implicated in Maple Syrup Urine Disease. Proline is cyclic, introduces kinks in peptide chains, and is essential for collagen.

Aromatic amino acids, containing benzene rings, are explored. Tryptophan has a double ring and is a precursor for niacin, serotonin, and melatonin. Phenylalanine is relatively nonpolar and converts to tyrosine. Tyrosine is relatively polar, derived from phenylalanine via phenylalanine hydroxylase (deficient in PKU), and a precursor for dopa, dopamine, norepinephrine, and epinephrine (catecholamines), as well as thyroid hormone and melanin.

This section covers polar non-aromatic amino acids. Serine and Threonine contain hydroxyl groups, with threonine also containing a methyl group and having two chiral centers. Cysteine contains sulfur, forming disulfide bonds important for protein structure. Asparagine and Glutamine contain amide groups, with glutamine notably important for nitrogen transport.

The video examines charged amino acids. Aspartate and Glutamate are negatively charged (acidic), with aspartate aiding nitrogen elimination in the urea cycle and glutamate being an excitatory neurotransmitter. Arginine, Lysine, and Histidine are positively charged (basic). Arginine has three nitrogens and produces nitric oxide (a vasodilator). Lysine forms carnitine for fatty acid transport. Histidine has an imidazole ring, acts as a physiological buffer, and is abundant in histones.

A summary of amino acid derivatives and their functions is presented. Tyrosine derivatives include catecholamines and thyroid hormone. Glycine contributes to porphyrin and heme. Tryptophan forms serotonin, melatonin, niacin, and NAD+. Histidine produces histamine through decarboxylation, a process often involving Vitamin B6. Glycine, proline, and lysine are crucial for collagen synthesis.

Protein metabolism involves building up (anabolism, peptide bond formation, condensation/dehydration reaction) and breaking down (catabolism, peptide bond breakdown, hydrolysis reaction). Peptide bonds are amide groups formed by the removal of water. Proteases (peptidases like trypsin and chymotrypsin) add water to break these bonds during digestion. Trypsin breaks bonds at the C-terminus of arginine and lysine, while chymotrypsin targets aromatic amino acids.

During prolonged starvation, the body breaks down proteins, particularly muscle proteins, as a last resort for energy (glucagon world). This process involves transamination and deamination of amino acids. The nitrogen components enter the urea cycle to be excreted as urea, while the remaining carbon skeletons can be converted to glucose (glucogenic amino acids) or ketone bodies (ketogenic amino acids) to provide energy.