Proteins Nitrogen balance Seminar No. 4 - Chapter 10 - Overall metabolism of proteins * Schemes on p. 53 and 54 * all proteins in the body are continuously degraded (metabolized) and newly synthesized * free AA from food, tissue proteins and non-essential AA from synthesis make AA pool * AA pool is used for: new body proteins specialized products (amines, NO, porphyrines, NA bases ...) catabolic proceses (energy gain) AA pool ~ 80 % in muscles ~ 10 % in liver ~ 5 % in kidney ~ 5 % in blood AA in blood Resorption phase * predominate Val, Leu, Ile * liver does not take them up from circulation (no specific aminotransferases in liver for Val, Leu, Ile) Postresorption phase and fasting * predominate Gln and Ala * released from muscles (Gln + Ala) and liver (Gln) There are eight essential aminoacids * valine * leucine * isoleucine * threonine * phenylalanine * tryptophan * lysine * methionine Biological value of some proteins Whey Catabolic pathways of AA *dietary proteins (r) AA (GIT, pepsin, trypsin etc.) *transamination of AA in cells (r) glutamate [*]dehydrogenation deamination of glutamate (r) NH[3 ]*detoxication of ammonia (r) urea, glutamine Q. (p. 54) Write a general equation of a reaction catalyzed by aminotransferases. General scheme of transamination Q. (p. 54) Which cofactor is used by aminotransferases? Pyridoxal phosphate 1. Phase of transamination 2. Phase of transamination In transaminations, nitrogen of most AA is concentrated in glutamate Dehydrogenation deamination of glutamate is a reversible reaction Intracellular localization Q. What are two main sources of ammonia in human body? Two main sources of ammonia in body * Dehydrogenation deamination of glutamate in cells of most tissues * Bacterial fermentation of proteins in large intestine ammonia diffuses freely into portal blood TH portal blood has high concentration of NH[3] TH NH[3] is eliminated by liver (under normal cond.) Other ways of deaminations * deamination of glutamate in purine nucleotide cycle (p. 55) * oxidative deamination of some AA ((r) H[2]O[2]) * oxidative deamination of biogenous amines ((r) H[2]O[2]) [* ] desaturation deamination of His (r) urocanic acid + NH[3 ]* oxidative deamination of lysinu lysyloxidase (Cu^2+): Lys + O[2] (r) NH[3] + allysine + H[2]O (p. 114) Oxidative deamination of some AA Oxidative deamination of amines Desaturation deamination of histidine Other sources of ammonia in tissues * non-enzymatic carbamylation of proteins Prot-NH[2] + NH[2]-CO-NH[2] (r) NH[3] + Prot-NH-CO-NH[2 ]* catabolism of pyrimidine bases cytosine/uracil (r) NH[3] + CO[2] + b-alanine thymine (r) NH[3] + CO[2] + b-aminoisobutyrate * hem synthesis 4 porfobilinogen (r) 4 NH[3] + uroporfyrinogen Three ways of ammonia detoxication Hydrolysis of arginine provides urea What is metabolic origine of N atoms in urea? The properties of urea * carbonic acid diamide * perfectly soluble in water * non-electrolyte TH neutral aq. solutions (pH = 7) * produced in liver × excreted by kidneys * difuses easily through all cell membranes Glutamine synthesis requires ATP Compare two reactions (GMD) Glucose-alanine cycle Glutamine cycle * Gln carries --NH[2] group from muscles to liver (periportal hepatocytes) for detoxification * in perivenous hepatocytes, Gln is made from glutamate to keep ammonia concentration low (Glu + NH[3] (r) Gln) * Gln is exported from the liver to kidney where is deaminated, NH[4]^+ ions are excreted by urine * exogenous and endogenous Gln is the source of energy for intestinal mucosa: Gln (r) 2-OG (r) energy (CAC) Glutamine deamination in kidney occurs stepwise Q. What reaction is catalyzed by glutaminase? Glutaminase catalyses the hydrolysis of amide group in glutamine Test results will be available on line is.muni.cz Spring 2006 Biochemistry II-s Notebook