Chapter 7 Amino Acid Metabolism The biochemistry and molecular biology department of CMU
§ 1.1 The significance of proteins 1. Keep the cells and tissues growing, renewing and mending 2. Take part in some kinds of important physiological activities 3. Oxidation and supply energy
§ 1.2 The requirements and nutritious value of proteins 1. Nitrogen balance Measuring the amount of intake and losses of total nitrogen can help us to know the general situation of protein metabolism.
There are three kinds of conditions: 1) Normal nitrogen balance intake N = losses N 2) Positive nitrogen balance intake N > losses N 3) Negative nitrogen balance intake N < losses N
2. Physical requirements of proteins • Lowest requirement: 30~50g/day • Recommend requirement: 80g/day (65kg man)
3. Nutrition value of proteins (1) Essential amino acids : Amino acids that cannot be synthesized by the body and must be obtained from the diet. Eight kinds of essential AAs: Val, Ile, Leu, Phe, Met, Trp, Thr, Lys
(2) Non- essential amino acids other 12 kinds of AAs (3) Semi-essential amino acids Tyr←Phe Cys←Met Note: His and Arg are essential AAs for infants and children.
(4) Complementary effect of dietary proteins • Two or more plant proteins are consumed together which complement each other in essential amino acid content.
§2.1 Digestion site: stomach, small intestine enzymes: pepsin Proteolytic enzymes of pancreatic juice
Proteolytic enzymes of pancreatic juice trypsin: Arg, Lys (C) chymotrypsin: Tyr, Trp, Phe, Met, Leu (C) endopeptidases elastase: Ala, Gly, Ser (C) carboxypeptidase exopeptidases aminopeptidase
enterokinase trypsin trypsinogen chymotrypsinogen chymotrypsin elastase proelastase procarboxypeptidase carboxypeptidase
§2.2 Absorption §2.3Putrefaction of proteins Concept: Some undigested proteins and no absorbed products are anaerobic decomposed by the bacteria in intestine. The products are toxic to body except few vitamin and fatty acid.
2. Production of ammonia (NH3) • Two sources: (1) Metabolism on unabsorbed amino acids (2) Urea hydrolyzed by urease
3. Some other toxic materials • Tyr → phenol • Trp → indole • Cys → hydrogen sulfide (H2S)
§ 3.1 The sources and fates of AAs Amino acid metabolic pool: amino acids in intracellular and extracellular fluids.
1. Sources of amino acids • Dietary protein from intestine • Breakdown of tissue protein • Synthesis in the body
2. Fates of amino acids • Deamination • Decarboxylation • Synthesis of non-protein nitrogen compounds such as purine and pyrimidine • Synthesis of proteins
§ 3.2 Degradation of protein in cells 1. Lysosomal pathway • Extracellular proteins, membrane-associated proteins and long-lived proteins • ATP-independent process • Cathepsins
2. Cytosol pathway • Abnormal proteins, damaged proteins and short-lived proteins • ATP and ubiquitin • Proteasome
§ 3.3 The catabolism of AAs 1. Deamination of AAs Four types: transamination oxidative deamination union deamination non-oxidative deamination
Transamination is the process by which an amino group, usually from glutamate, is transferred to an α-keto acid, with formation of the corresponding amino acid plus α-ketoglutarate.
Key points: ① reversible ② Lys and Pro cannot be transaminated. ③ Aminotransferases utilize a coenzyme - pyridoxal phosphate - which is derived from vitamin B6.
Amino acid pyridoxal phosphate Schiff base pyridoxamine phosphate α-keto acid Isomer of Schiff base
Two important transaminases: ALT: Alanine aminotransferase (in liver) AST: Aspartate aminotransferase (in heart)
(3) Union deamination The α- amino group of most amino acids is transferred to α- ketoglutarate to form an α- keto acid and glutamate by transaminase. Glutamate is then oxidatively deaminated to yield ammonia and α- ketoglutarate by glutamate dehydrogenase.
Pyruvate + glutamate Alanine + α-ketoglutarate Glutamate + NAD+ + H2O α-ketoglutarate + NADH + NH4+ Net Reaction: Alanine + NAD+ + H2O pyruvate + NADH + NH4+
2. Metabolism of -keto acid (1) Formation of non- essential AAs (2) Formation of glucose or lipids (3) Provide energy
catabolites of amino acid -Ketoglutarate Succinyl CoA Intermediates of TAC Fumarate Oxaloacetate PEP Glucose Pyruvate Fatty acid Acetyl CoA Acetoacetyl CoA Ketone bodies
Amino acids of converted into ketone bodies or fatty acids are termed ketogenic amino acids. Amino acids of converted into glucose are termed glucogenic amino acids. Amino acids of converted into both glucose and ketone bodies are termed glucogenic and ketogenic amino acids.
§ 4.1 Source and outlet of ammonia (NH3) 1. Sources: ⑴ Endogenous sources: ①Deamination of AAs--main source ② Catabolism of other nitrogen containing compounds ③ Kidney secretion (Gln)
⑵ Exogenous sources： ① Putrefaction in the intestine. ② Degradation of urea in the intestine
2. Outlets: (1) Formation of urea (2) Formation of Gln (3) Excrete in urine (4) Synthesis of AA
§ 4. 2 Transportation of NH3 1. Alanine-glucose cycle 2. Transportation of ammonia by Gln
§ 4. 3 Formation of urea 1. Site: liver (mitochondria and cytosol) 2. Process --------- ornithine cycle