Proteins Summary & MCQs

1. Definition & Composition • Proteins = Large nitrogenous macromolecules • Made of amino acids (only 20 used in mammalian protein synthesis) • Contain C, H, O, N, sometimes S, P, Fe, I, Zn, Cu 2. Amino Acids (AA) – Basics Each AA contains: • –NH₂, –COOH, H, and R group (side chain) Sajid Afridi Lecturer SHS Mnemonic for Structure: "NH-CO-H-R" (Nitrogen, Carbonyl, Hydrogen, R side group) 3. Classification of Amino Acids A. Neutral AAs • Aliphatic: Gly, Ala, Val, Leu, Ile, Ser, Thr • Aromatic: Phe, Tyr • Heterocyclic: Trp, His • Imino Acids: Pro, Hydroxyproline • Sulfur-containing: Met, Cys → Cystine (via disulfide bond) B. Acidic AAs • Aspartic acid, Glutamic acid Mnemonic: "Acids Are Good" → Asp, Glu C. Basic AAs • Arginine, Lysine, Hydroxylysine Mnemonic: "HAL is Basic" → His, Arg, Lys 4. Nutritional Types of AAs Type Examples Essential Val, Leu, Ile, Phe, Thr, Trp, Met, Lys MATT VIL PHLY Non-essential Synthesized by body Mnemonic Gly, Ala, Glu, Asp, etc. Semi-essential Required during growth/pregnancy Arginine, Histidine 5. Special AAs (Non-standard) • Ornithine, Citrulline: Urea cycle • GABA: From Glutamate • Thyroxine, T3: From Tyrosine • DOPA: Melanin precursor • SAM: Methyl donor from Methionine 6. Amphoteric & Zwitterions • AAs act as acid + base → amphoteric • At neutral pH, exist as zwitterion • Isoelectric point (pI): Net charge = 0 Zwitterion = Dual ion = neutral at pI 7. Protein Classification A. Based on Shape • Fibrous: Collagen, Keratin (long, strong) • Globular: Hemoglobin, Enzymes (compact, round) B. Based on Function • Enzymes, Hormones, Transport (Hb), Defense (Ig), Structure (collagen) C. Based on Solubility • Simple: Albumin, Globulin, Prolamines • Conjugated: Lipoproteins, Glycoproteins, Nucleoproteins • Derived: Denatured (heat/chemicals), hydrolyzed 8. Peptide Bond & Peptides • Peptide bond: Between –COOH and –NH₂ of two AAs • Oligopeptides: Few AAs (2–10) • Polypeptides: Many AAs • Protein: Usually >10,000 Da Tripeptide = 3 AAs, 2 bonds; Tetrapeptide = 4 AAs, 3 bonds 9. Protein Structure – 4 Levels Level Bonds Involved Mnemonic Primary Peptide bonds "1-Peptide" Secondary H-bonds, disulfide "2-Stable Spirals/Sheets" Tertiary "3-Folds & Functions" Disulfide, Ionic, H Quaternary Subunit interactions "4-Subunit Structure" Mnemonic: "1-Peptide, 2-H-bond, 3-Fold, 4-Join" 10. Clinical & Functional Roles of AAs • Tyrosine → Thyroid hormones, melanin, catecholamines • Tryptophan → Serotonin, Niacin • Histidine → Histamine • Cysteine, Glycine → Bile salts, Glutathione • Glutamate → Neurotransmitters Quick Flashcards Term Definition / Role Zwitterion Equal +ve and –ve charges at neutral pH Isoelectric pH pH where net charge = 0 Fibrous Protein Long, insoluble (e.g., collagen) Globular Protein Compact, soluble (e.g., enzymes, Hb) Peptide Bond –COOH + –NH₂ → bond + H₂O Here are 50 Standard MCQs from the Proteins chapter (based on your provided PPT and Satyanarayana Biochemistry content). Each question includes 4 options with the correct answer highlighted. MCQs – Proteins (50 Questions) 1. Proteins are composed primarily of which elements? a) C, H, O b) C, H, O, N c) C, H, O, N, S d) C, H, O, P, Mg 2. The building blocks of proteins are: a) Fatty acids b) Nucleotides c) Amino acids d) Monosaccharides 3. Which amino acid is classified as imino acid? a) Glycine b) Proline c) Glutamine d) Alanine 4. The peptide bond is formed between: a) R groups of amino acids b) Carbon and hydrogen c) –COOH and –NH₂ groups d) Side chains only 5. How many standard amino acids are used in mammalian proteins? a) 16 b) 25 c) 20 d) 18 6. Which amino acid is sulfur-containing? a) Serine b) Methionine c) Histidine d) Asparagine 7. Which is an aromatic amino acid? a) Valine b) Phenylalanine c) Threonine d) Arginine 8. Which amino acid gives rise to niacin? a) Tyrosine b) Tryptophan c) Histidine d) Glutamine 9. Which amino acid forms thyroid hormones? a) Tyrosine b) Methionine c) Glycine d) Alanine 10. Histamine is derived from which amino acid? a) Serine b) Histidine c) Glycine d) Tryptophan 11. An amino acid acts as both acid and base. It is said to be: a) Neutral b) Amphoteric c) Aromatic d) Aliphatic 12. The neutral form of an amino acid at physiological pH is called: a) Ionic form b) Zwitterion c) Covalent form d) Ligand 13. The isoelectric point (pI) is: a) The highest pH of an amino acid b) The pKa of the amino group c) The pH at which the net charge is zero d) The basic property of amino acid 14. Which of the following is a basic amino acid? a) Tyrosine b) Glutamic acid c) Lysine d) Threonine 15. Which amino acid contains an indole ring? a) Phenylalanine b) Tryptophan c) Tyrosine d) Histidine 16. Cystine is formed from two molecules of: a) Cysteine b) Methionine c) Glycine d) Alanine 17. Which amino acid is classified as semi-essential? a) Arginine b) Lysine c) Leucine d) Valine 18. What is the prosthetic group of hemoglobin? a) DNA b) Protein c) Heme d) Glucose 19. Which protein is found in connective tissue and bone? a) Elastin b) Keratin c) Collagen d) Histone 20. α-Keratin is rich in which amino acid? a) Cysteine b) Glycine c) Serine d) Valine 21. Which protein is an example of a conjugated protein? a) Albumin b) Hemoglobin c) Myosin d) Keratin 22. Which of the following is NOT a function of proteins? a) Enzyme catalysis b) Storage of genetic material c) Hormone function d) Transport 23. Which protein carries oxygen in muscles? a) Collagen b) Myoglobin c) Hemoglobin d) Elastin 24. Simple proteins yield: a) Only amino acids b) Fatty acids c) Carbohydrates d) Vitamins 25. Histones are rich in: a) Arginine and Histidine b) Tyrosine and Glycine c) Valine and Isoleucine d) Cysteine and Serine 26. Globular proteins are: a) Soluble and compact b) Insoluble and elongated c) Only in plants d) Always fibrous 27. Which amino acid has the smallest side chain? a) Glycine b) Alanine c) Serine d) Valine 28. Which is NOT an aromatic amino acid? a) Tyrosine b) Serine c) Tryptophan d) Phenylalanine 29. Which bond is responsible for secondary structure? a) Hydrogen bonds b) Peptide bonds c) Ionic bonds d) Ester bonds 30. Primary structure of a protein is: a) Folding of chains b) Sequence of amino acids c) Beta sheets d) Disulfide linkage 31. Which of the following proteins is fibrous? a) Hemoglobin b) Collagen c) Albumin d) Insulin 32. Which is a conjugated protein with metal? a) Ferritin b) Albumin c) Casein d) Globulin 33. Albumin is an example of: a) Simple globular protein b) Conjugated protein c) Fibrous protein d) Derived protein 34. Peptones are: a) Secondary derived proteins b) Simple proteins c) Hormones d) Fibrous proteins 35. Disulfide bonds occur between: a) Alanine molecules b) Cysteine residues c) Glycine and proline d) Glutamate and arginine 36. Glutathione is composed of: a) Glutamate, Cysteine, Glycine b) Arginine, Lysine, Histidine c) Methionine, Tyrosine d) Proline, Serine 37. Protamines are basic proteins found in: a) Fish sperm b) Human hair c) Muscle d) Connective tissue 38. Which protein is highly stable and forms support structures? a) Albumin b) Scleroproteins (e.g., keratin) c) Globulin d) Casein 39. Which AA can donate methyl groups? a) Phenylalanine b) Serine c) Methionine (via SAM) d) Glycine 40. Glycoproteins have: a) Sugar as prosthetic group b) DNA backbone c) Fatty acid as prosthetic group d) Only protein 41. Casein is an example of: a) Glycoprotein b) Phosphoprotein c) Lipoprotein d) Conjugated enzyme 42. Enzyme that breaks down H₂O₂: a) Peroxidase b) Catalase c) Superoxide dismutase d) Trypsin 43. Which amino acid is rich in yellow elastic fibers? a) Tyrosine b) Glycine c) Proline and Valine d) Histidine 44. Which of these is found in ribosomes? a) DNA-protein only b) Ribonucleoproteins c) Lipoproteins d) Metalloproteins 45. Which of the following proteins contains a carotenoid pigment? a) Visual purple b) Catalase c) Casein d) Elastin 46. HDL is a type of: a) Enzyme b) Lipoprotein c) Hormone d) Vitamin 47. Proteins below 10,000 Da are called: a) Polysaccharides b) Polypeptides c) Glycans d) Scleroproteins 48. Which type of protein includes mucins and immunoglobulins? a) Conjugated only b) Glycoproteins c) Lipoproteins d) Simple proteins 49. The quaternary structure is formed by: a) Folding within one chain b) Association of multiple chains c) Peptide linkage d) Hydrogen bonding only 50. The end of a polypeptide chain with free –NH₂ is called: a) N-terminal b) C-terminal c) Alpha terminal d) Amino terminal Here are mnemonics to help you memorize amino acids and their one-letter and threeletter symbols, grouped logically for easy learning. Amino Acids Mnemonics (With Symbols) 1. Nonpolar (Hydrophobic) Amino Acids Mnemonic: "Grandma Always Visits London In May For Winston’s Party" (Nonpolar → tend to be buried inside proteins) Amino Acid 3-Letter 1-Letter Glycine Gly G Amino Acid 3-Letter 1-Letter Alanine Ala A Valine Val V Leucine Leu L Isoleucine Ile I Methionine Met M F (Phenylalanine) Phe F W (Tryptophan) Trp W Proline Pro P 2. Polar Uncharged Amino Acids Mnemonic: "Santa’s Team Crafts New Quilts Yearly" (These can form hydrogen bonds, no net charge) Amino Acid 3-Letter 1-Letter Serine Ser S Threonine Thr T Cysteine Cys C NAsparagine Asn N QGlutamine Gln Q YTyrosine Y Tyr 3. Acidic Amino Acids (Negatively Charged) Mnemonic: "Acids are EDgy" E → Glutamic acid, D → Aspartic acid Amino Acid 3-Letter 1-Letter DAspartic acid Asp D EGlutamic acid Glu E 4. Basic Amino Acids (Positively Charged) Mnemonic: "His Lies Are Basic" H → Histidine, K → Lysine, R → Arginine Amino Acid 3-Letter 1-Letter Histidine His H KLysine Lys K RArginine Arg R 5. Essential Amino Acids Mnemonic: "PVT TIM HALL" Essential AA 3-Letter 1-Letter Phenylalanine Phe F Valine Val V Threonine Thr T Tryptophan Trp W Isoleucine Ile I Methionine Met M Histidine His H Arginine* Arg R Essential AA 3-Letter 1-Letter Leucine Leu L Lysine Lys K *Arginine is semi-essential Bonus Tip: Alphabet Trick Some 1-letter codes match the first letter: • Alanine → A • Cysteine → C • Glycine → G • Histidine → H • Isoleucine → I • Leucine → L • Methionine → M • Proline → P • Serine → S • Threonine → T • Valine → V