Biochemistry - 43 -Special Products Derived from Amino Acids 2023.pdf

Glutathione (GSH) Objective A • Glutathione (γ-glutamylcysteinylglycine) is one of the body’s principal means of protecting against oxidative damage. • Tripeptide composed of glutamate, cysteine, and glycine: synthesized in two steps (1) gammaglutamylcysteine synthase and (2) glutathione synthase...

Glutathione (GSH) Objective A • Glutathione (γ-glutamylcysteinylglycine) is one of the body’s principal means of protecting against oxidative damage. • Tripeptide composed of glutamate, cysteine, and glycine: synthesized in two steps (1) gammaglutamylcysteine synthase and (2) glutathione synthase (all non-essential amino acids) • Reactions are catalyzed by glutathione peroxidases • Reactive sulfhydryl groups reduce hydrogen peroxide to water and lipid peroxides to nontoxic alcohols • 2 GSH molecules are oxidized to form glutathione disulfide • -SH groups are also oxidized in non- enzymatic chain terminating reactions with organic radicals. • Primarily located in the cytosol and mitochondria • Major means of removing H2O2 (protection against free radical injury). • Oxidized GSSG must be reduced back to sulfhydryl form by glutathione reductase in a redox cycle. 4 Glutathione (GSH) Objective A 5 Creatine Objective B Synthesis • Creatine synthesis begins in the kidney and ends in the liver • In the kidney, glycine combines with arginine to form guanidinoacetate • Guanidinium group of arginine is transferred to glycine and the remainder of the arginine is released as ornithine. • Guanidinoacetate travels to the liver where it is methylated by S-adenosylmethionine (SAM) to form creatine 6 Creatine Phosphate Objective B Function – reservoir of high energy phosphate • Creatine travels from liver to other tissues (particularly muscle & brain) where it is reversibly phosphorylated to creatine phosphate by creatine kinase, using ATP as the donor. • Creatine phosphate is now a high-energy compound that serves as small reserve of high-energy phosphates that can be mobilized when needed. • These high-energy phosphates can be reversibly transferred to ADP to maintain (regenerate) intracellular levels of ATP needed for muscle contraction. Degradation • Creatine phosphate is unstable and spontaneously cyclizes to creatinine which cannot be further metabolized (excreted in urine) • Amount of creatinine excreted is proportional to the total creatine phosphate content of the body and, therefore, can be used to estimate muscle mass • When muscle mass decreases (i.e. paralysis or muscular dystrophy) the creatinine content of the urine decreases • In addition, any rise in blood creatinine is a sensitive indicator of kidney malfunction 7 Porphyrin Metabolism Heme Objective C • Porphyrins are cyclic compounds that readily bind metal ions, usually ferrous (Fe2+) or ferric (Fe3+) iron • The most prevalent metalloporphyrin in humans is heme • Heme consists of one Fe2+ coordinated in the center of a tetrapyrrole ring of protoporphyrin IX. • Side chains are methyl (-CH3), vinyl (-CH=CH2), and propionyl (-CH2-CH2-COO-) • Heme is the prosthetic group for hemoglobin, myoglobin, the cytochromes, the cytochrome P450 monooxygenase system, catalase, nitric oxide synthase, and peroxidase. 9 Lead Poisoning Objective D • δ-ALA dehydratase and ferrochelatase are inactivated by lead. • Both of these enzymes contain zinc which makes them extremely sensitive to inhibition by heavy metal ions (e.g. lead) that replace the zinc. Result: • δ-ALA and protoporphyrin IX accumulate… heme decreased. • This inhibition is, in part, responsible for some of the characteristics of lead poisoning • • Anemia results from lack of hemoglobin Energy production decreases because of lack of cytochromes for electron-transport chain. 11 Porphyrias Objective D Porphyrias are a group of rare, inherited (or occasionally acquired) defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors. • Each porphyria results in the accumulation of a unique pattern of intermediates caused by the deficiency of an enzyme in the heme synthetic pathway. • Intermediates (porphyrinogens) of the pathway accumulate and have toxic effects on the nervous system. • Porphyrinogens may be converted by light to porphyrins, which react with O2 to form oxygen radicals. • Can cause severe damage to the skin • Photosensitivity Objective d 12 Tyrosine: Catecholamines Objective J • Dopamine, norepinephrine, and epinephrine are biologically active amines that are collectively termed catecholamines • Dopamine and norepinephrine are synthesized in the brain and function as neurotransmitters • Norepinephrine is also synthesized in the adrenal medulla, as is epinephrine Function: • Outside the CNS, norepinephrine and epinephrine are hormone regulators of carbohydrate and lipid metabolism • Norepinephrine and epinephrine are released in response to fright, exercise, cold, and low levels of blood glucose. • They increase the degradation of glycogen and triacylglycerol as well as increase blood pressure and cardiac output • These effects are part of the coordinated response to prepare an individual for stress and are often called the “fight-of-flight” response. 23 Catecholamines Objective J • All catecholamines are synthesized in a common pathway from tyrosine • 1st step (rate-limiting) in synthesis from tyrosine is the hydroxylation of the tyrosine ring by tyrosine hydroxylase [BH4 requiring enzyme] to form DOPA • DOPA is decarboxylated to form dopamine [PLP required] • Dopamine is hydroxylated by dopamine β-hydroxylase to yield norepinephrine in a reaction that requires Vitamin C (Ascorbate) and copper • Epinephrine is formed from norepinephrine by an N-methylation reaction using SAM as the methyl donor Objective j 24 Catecholamines Objective J Degradation • The action of catecholamines is terminated through reuptake and degradation in the presynaptic terminal and in adjacent cells • The catecholamines are inactivated by oxidative deamination catalyzed by monoamine oxidase (MAO) and by O-methylation by catechol-O-methyltransferase (COMT) • The two reactions can occur in either order • MAO is present on the outer mitochondrial membranes of many cells and oxidizes the amino group to an aldehyde releasing ammonium ion. • MAO inactivates catecholamines that are not protected in storage vesicles • 2 isoforms of MAO • MAO-A preferentially deaminates norepinephrine and serotonin • MAO-B acts on a wide spectrum of phenylethylamines • The aldehyde products of these reactions are excreted in the urine as vanillylmandelic acid (VMA) from epinephrine and norepinephrine and homovanillic acid from dopamine Objective j 25 Parkinson’s Disease Objective K (Review from Toxic Oxygen lecture) • Major clinical disturbances in Parkinson’s are a result of dopamine depletion in the neostriatum, resulting from degeneration of dopaminergic neurons • Decrease in dopamine production is the result of severe degeneration of these neurons. • Unknown initiation agent, but various studies implicate mitochondrial dysfunction (free radicals) What is the role of MAO in Parkinson’s? • Dopamine levels are reduced by MAO which generates superoxide (H2O2) ß ROS • NO (produced by iNOS) can react with superoxide to form RNOS • RNOS and hydroxyl radical can lead to radical chain reactions that result in lipid peroxidation, protein oxidation, and neuronal degeneration. • The end result is a reduced production and release of dopamine which leads to clinical symptoms • Deprenyl (irreversible MAO-B inhibitor) is used to treat Parkinson's 26 Melanin Objective L • Melanin is a pigment that occurs in several tissues, particularly the eye, hair, and skin. • It functions to protect underlying cells from harmful effects of sunlight • It is synthesized from tyrosine in melanocytes (pigment-forming cells) of the epidermis • Tyrosine is hydroxylated by a copper containing isozyme tyrosine hydroxylase to form DOPA • DOPA is oxidized to quinones which polymerize to form melanin pigments • Albinism (hypomelanosis) is caused by defective Cu-dependent tyrosine hydoxylase (of melanocytes) –or- other enzymes that convert Tyr to melanins • Albinism patients suffer from lack of pigment (hypopigmentation) in the skin, hair, and eyes, and they are sensitive to sunlight. 27 Sample Question Which molecule functions as the high energy phosphate reservoir in muscle? A. creatine B. creatine kinase C. creatine phosphate D. creatinine E. glycine 28 Sample Question A patient has malignant phenylketonuria. Because of this disorder, which of molecule or molecules cannot be synthesized by the patient either? A. Dopamine B. Dopamine, Epinephrine and Norepinephrine C. Epinephrine and Norepinephrine D. Epinephrine E. Norepinephrine 29