《生物化学》课程PPT教学课件（英文版）Chapter 18 Amino acid Oxidation and the Production of Urea

hapter Amino Acid Oxidation and the Production of urea

Chapter 18 Amino Acid Oxidation and the Production of Urea

I. The surplus amino acids in animals can be completely oxidizer or converted to other storable fuels Amino acids in excess(from diet protein turnover can neither -be stored, nor excreted, but oxidized to release energy or converted to fatty acids or glucose Animals also utilize amino acid for energy generation during starvation or in diabetes mellitus Microorganisms can also use amino acids as an energy source when the supply is in excess Plants almost never use amino acids as an energy source(neither fatty acids)

1. The surplus amino acids in animals can be completely oxidized or converted to other storable fuels • Amino acids in excess (from diet, protein turnover) can neither be stored, nor excreted, but oxidized to release energy or converted to fatty acids or glucose. • Animals also utilize amino acid for energy generation during starvation or in diabetes mellitus. • Microorganisms can also use amino acids as an energy source when the supply is in excess. • Plants almost never use amino acids as an energy source (neither fatty acids)

2. Dietary proteins are digested into amino acids in the gastrointestinal ract Pepsin cleaves polypeptides into smaller peptides in stomach(N-terminal side ofY, F, W residues) Trypsin( C-terminal side of K, R) and chymotrypsin (C-terminal side of F, W, and Y)further cleave the peptides in small intestine Carboxypeptidase and aminopeptidase cleave the small peptides into amino acids. which are then absorbed and eventually delivered to liver

2. Dietary proteins are digested into amino acids in the gastrointestinal tract • Pepsin cleaves polypeptides into smaller peptides in stomach (N-terminal side of Y, F, W residues). • Trypsin (C-terminal side of K, R) and chymotrypsin (C-terminal side of F, W, and Y) further cleave the peptides in small intestine. • Carboxypeptidase and aminopeptidase cleave the small peptides into amino acids, which are then absorbed and eventually delivered to liver

stomach lining Parietal cells (secrete HCD (secrete pepsinogen) Stomach Gastric mucosa (secretes gastrin) Pancreas ancreatic d Exocrine cells of pancreas Zy coll ct and exopeptidase Villi of small intestine Small Vill Intestine Intestinal (absorbs amino

Pepsin Chymotrypsin,trypsin, and exopeptidases Amino acids

3. The amino groups and carbon skeletons of amino acids, take separate but interconnected path ways The amino group is reused or excreted, as ammonia urea(via the urea cycle) or uric acid The carbon skeletons(a-keto acids) generally find their way to the citric acid cycle for further oxidation or conversion. The degradation of the carbon skeletons may be very complicated but similar to that of fatty acids in some cases

3. The amino groups and carbon skeletons of amino acids take separate but interconnected pathways • The amino group is reused or excreted, as ammonia, urea (via the urea cycle) or uric acid. • The carbon skeletons (a-keto acids) generally find their way to the citric acid cycle for further oxidation or conversion. • The degradation of the carbon skeletons may be very complicated but similar to that of fatty acids in some cases

Intracellular protein Dietary protein Amino acids NH Carbon skeletons Biosynthesis of amino acids, 4/ nucleotides, and biological amines v Carbamoyl phosphate a-Keto acids Aspartate- arginine- Urea Citric succinate cle acid CO2+H2O shunt of cycle ATE citric acid cle Oxaloacetate U (nitrogen excretion product) Glucose (synthesized in gluconeogenesis)

4. Liver is the maior site of amino acid degradation in vertebrates. a-ketoglutarate(from amino acids ), glutamate (from free ammonia), pyruvate (from muscle amino acids) collects amino groups (in forms of Glu, Gln and Ala to liver mitochondria for further processing The excess NH4+ is excreted directly in bony fishes as urea in most terrestrial vertebrates. or as uric acid in birds and terrestrial reptiles. 3 The carbons in both urea and uric acid are highl oxidized (with most of the energy extracted

4. Liver is the major site of amino acid degradation in vertebrates a-ketoglutarate (from amino acids), glutamate (from free ammonia), pyruvate (from muscle amino acids) collects amino groups (in forms of Glu, Gln and Ala) to liver mitochondria for further processing. • The excess NH4+ is excreted directly in bony fishes, as urea in most terrestrial vertebrates, or as uric acid in birds and terrestrial reptiles. • The carbons in both urea and uric acid are highly oxidized (with most of the energy extracted)

llular protein COo COo Amino acids from ingested→HaN-(-H p R R Amino acids a-Keto acids COO H3N-C-H CH CH. COO COo a-Ketoglutarate Glutamate Coo HN H NH4 cOo COo Glutamine fro Alanine muscle and fr H3N-C-H other tissue CH CH o NH2 Alanine Pyruvate Glutamine NH 4, urea, or uric acid

O H C HIN C NHA H2N--C-NH2 C=0 C Ammonia (as N ammonium ion) O H rea H Uric acid Ammonotelic animals: most aquatic vertebrates Ureotelic animals such as bony fishes and many terrestrial Uricotelic animals: the larvae of amphibia vertebrates; also sharks birds, reptiles

5.PLP facilitates the transamination and other transformations of amino acids Different aminotransferases(e. g. aspartate and alanine aminotransferases), each catalyzes the transfer of the amino group from an amino acid to a-ketoglutarate to form Glu and a a-keto acid Pyridoxal phosphate(PLP), being derived from vitamin B (i. e, pyridoxine) and the prosthetic oups for all the aminotransferases, act as a temporary carrier of the amino groups PLP accepts and then donates an amino group by forming a Schiff base with the amino-donating amino acid and amino-accepting a-keto acid(being a-ketoglutarate in many cases respectively

5. PLP facilitates the transaminatin and other transformations of amino acids • Different aminotransferases (e.g., aspartate and alanine aminotransferases), each catalyzes the transfer of the amino group from an amino acid to a-ketoglutarate to form Glu and a a-keto acid. • Pyridoxal phosphate (PLP), being derived from vitamin B6 (i.e., pyridoxine) and the prosthetic groups for all the aminotransferases, act as a temporary carrier of the amino groups. • PLP accepts and then donates an amino group by forming a Schiff base with the amino-donating amino acid and amino-accepting a-keto acid (being a-ketoglutarate in many cases )respectively