Biochemistry II 2014 Step 1 Review

1. Biochemistry II2014 Step 1 Review Wednesday, January 11th Seth Wander sawander@med.miami.edu

2. Vitamin deficiencies • Collagen disorders • Chronic granulomatous disease • Metabolic disorders • Hemoglobin physiology • Sample questions

3. Vitamin Deficiencies Vit. A  night blindness, visual disturbances B1 (Thiamine)  Wernicke-Korsakoff, Beriberi B3 (Niacin)  Pellagra B12 (Cobalamin)  macrocytic, megaloblastic anemia + paresthesia Folate  macrocytic, megaloblastic anemia (no paresthesias); neural tube defects Vit. D  rickets, osteomalacia, tetany Vit. E  hemolytic anemias (oxidative sensitivity) Vit. K  neonatal hemorrhage

4. Disorders of Collagen Structure Collagen = most abundant protein in the human body, critical structural component of ECM, made @ fibroblasts. Basic peptide structure = Gly – X - Y • Synthesis (with clinical correlates): • 1) Translation of α chains @ RER, preprocollagen • 2) Hydroxylation of proline/lysine residues • Requires Vitamin C, deficiency = scurvy • 3) Glycosylation of lysine residues, formation of procollagen (triple helix) • Defective in osteogenesis imperfecta • 4) Exocytosis in extracellular space • 5) Proteolytic processing – terminal cleavage events generate insoluble tropocollagen • 6) Cross-linking – covalent association of tropocollagen molecules to generate mature collagen fibrils • * Defective in Ehlers-Danlos syndome Vitamin C, scurvy OH OH OI OH OH OH OH OH OH OH OH Ehlers-Danlos

5. Disorders of Collagen Structure • Recognizing clinical presentation: • Scurvy: look for history of vitamin C deficiency. • + swollen gums, bruising, anemia (Fe2= absorption), splinter hemorrhages, poor wound healing • OsteogenesisImperfecta: brittle bone disease, multiple fractures in children w/ minimal trauma (minimal soft tissue injury) • + blue sclerae, hearing loss • *may be confused with child abuse! • Ehlers-Danlos syndrome: easy bruising, hypermobile joints, hyperextensible skin • *associated with berry aneurysms Vitamin C, scurvy OH OH OI OH OH OH OH OH OH OH OH Ehlers-Danlos

6. Oxygen-Dependent Respiratory Burst • Chronic Granulomatous Disease = NADPH Oxidase deficiency • Recurrent infections with catalase (+) bugs! • Catalase (-) bugs (eg. strep) replace H2O2, allowing host defense even in CGD pts. Active Phagocytosis Phagolysosome • Catalase (+) bugs: • Staph • Aspergillus • Candida • Salmonella • Nocardia O2 NADPH NADPH Oxidase NADP+ O2· Superoxide Dismutase NADP+ NADPH Glucose-6-P dehydrogenase (HMP Shunt) H2O2 Cl Myeloperoxidase • G6PD deficiency = impairs pentose phosphate pathway NADPH production • Most common human enzyme deficiency • Malaria resistance • Hemolytic anemia due to RBC destruction by free radicals • Heinz bodies + bite cells HOCl · Neutrophils/macrophages

7. Homocystinuria Cystathionine synthase Methionine Homocysteine Cystathionine Cysteine B6 Methionine synthase B12 THF CH3 THF • Disease biochemistry: (All AR) • Cystathioninesynthase deficiency • Decreased affinity of cystathioninesynthase for B6 • Methioninesynthase deficiency • Clinical symptoms: • Mental retardation • Osteoporosis • Tall stature • Kyphosis • Lens subluxation • Atherosclerosis • Increased urine homocysteine

8. Phenylalanine metabolism Phenylalanine Tyrosine Homogentisic Acid Maleylacetoacetate Phenylalanine Hydroxylase Homogentisate Oxidase Fumarate • Phenylketonuria (AR): • Tyrosine becomes essential • Mental retardation • Growth retardation • Eczema • Musty body odor • ** Teratogenic! • Alkaptonuria/Ochronosis (AR): • Defective tyrosine degradation • Black urine on standing • Arthralgias TCA

9. Hemoglobin Physiology β1 β2 Relaxed (R) Taut (T) *High oxygen affinity (300X) (Tight binding) *Low oxygen affinity (O2 unloading) α2 α1 100 Hb saturation (%) 25 25 65 100 PO2 (mm Hg)

10. Hemoglobin Physiology Myoglobin Fetal Hb • Favors T form = Increased O2 unloading in metabolically active tissues • ↑ Temperature • ↑ H+ • ↓ pH • ↑ PCO2 • ↑ 2,3-BPG (2,3-DPG) Tissues Bohr Shift 100 Lungs Hb saturation (%) 25 25 65 100 PO2 (mm Hg)

11. Hemoglobin Physiology • Hemoglobin modifications: • Methemoglobin – Oxidixed form (ferrous Fe2+ ferric Fe3+) – Impaired oxygen carrying capacity! • - Multiple causes (including G6PDH deficiency!) • - Induced in order to treat cyanide poisoning (methemoglobin has higher CN- affinity). • - Methemoglobinemia treated with methylene blue • Carboxyhemoglobin – Hb bound to CO rather than O2 – Reduced oxygen carrying and unloading capacity! • ** CO has much greater (200X) affinity for Hb compared to O2 • CO2 transport: • Bicarbonate (90%) – converted from CO2 within peripheral tissues by carbonic anhydrase, reverse reaction occurs at lungs • Carbaminohemoglobin (5%) – bound at Hb N-terminus, not heme! • Dissolved CO2 (5%)

12. 1) A young child presents after repeated fractures following only minimal trauma. She also has congenital hearing loss and a slight blue discoloration of her sclerae. Which of the following accurately describes the abnormal biochemical process underlying this disease? A) Impaired collagen hydroxylation B) Impaired collagen glycosylation C) Defective covalent cross-linking of tropocollagen D) Vitamin C deficiency

13. 2) A child is seen due to complaints of recurrent pyogenic infections, especially with Staph. aureus. Upon physical exam, the child is noted to have very light, partially depigmented skin and loss of sensation in the extremities. What is the likely diagnosis? A) Osteogenesis Imperfecta B) Xeroderma Pigmentosa C) Cystic Fibrosis D) Chediak-Higashi Syndome

14. 3) Compared to patients with chronic granulomatous disease, individuals with myeloperoxidase deficiencies experience more frequent and more severe infections. Which of the following best explains this phenotype? A) Two separate phagocytic pathways are impaired in this scenario B) Myeloperoxidase acts downstream of H2O2 in the generation of free radicals, thus these patients are susceptible to both catalase (+) and (-) pathogens C) Mutations in myeloperoxidase cause inhibition of the pentose phosphate pathway, thus NADPH cannot be generated for the production of free radicals D) Myeloperoxidase functions in a greater variety of immune cells, thus loss of this enzyme impairs a wider range of protective functions

15. 4) A laboratory scientist separates a mixture of cellular protein utilizing gel electrophoresis. She then transfers the separated proteins to a membrane, and probes them with a specific combination of primary and secondary antibodies. Which of the following procedures does this describe? A) Southern blot B) Northern blot C) Western blot D) ELISA (Enzyme-Linked Immunosorbent Assay)

16. 5) A patient presents with atherosclerotic disease, decreased bone density, and lens subluxation. Upon physical exam you note that the patient is tall with a kyphotic posture. Urinalysis shows elevated homocysteine. This patient may benefit from dietary supplementation of which vitamin? A) C B) B2 (niacin) C) E D) B6 (pyridoxine)

17. 6) Which of the following metabolic changes would be expected to facilitate oxygen unloading in peripheral tissues? A) ↑ pH B) Reduced glycolysis C) ↓ H+ D) ↑ PCO2

18. 7) Identify the curve that represents cooperative binding between two alpha and two beta globin chains occurring within a tissue that demonstrates near maximal PO2. A B C D 100 Hb saturation (%) 25 25 65 100 PO2 (mm Hg)

19. 8) Phenylketonuria, alkaptonuria, and albinism are all due to defects in the metabolism of which amino acid? A) Tryptophan B) Phenylalanine C) Histidine D) Lysine