ACS Biochemistry Exam Questions With Complete Solutions Metabolism (catabolism and anabolism) - ANSWERS Metabolism: sum of total chemical reactions in an organism, also the m ethod by which cells extract and use energy from their environment. Catabolism: The process by which stored nutrients and ingested foods are converted to a usable form of energy. It produces simple products CO2, H2O, NH3, and building blocks such as suga rs and fats that are used in anabolism. Anabolism: the process by which simple products and building blocks of catabolism are used to create complex biological products that contribute to organismal growth and development. It also uses the energy produce d in catabolism to do biological work. Properties of cells - ANSWERS Metabolism: undergoing catabolic and anabolic processes. Reproduction: cell populations grow via asexual reproduction. Mutation: during growth and reproduction, cells someti mes make mistakes, leading to mutations and evolution. Respond to environment: metabolic pathways respond to signals, including light, touch, hormones, and nutrients, that can turn the pathways on or off. Speed and efficiency: cell operations are highl y specific to maximize targeting and efficiency. Similar building blocks: most species are very similar at the cellular level. What accounts for water's unique properties? - ANSWERS Hydrogen bonding The unique properties of water (specific heat, heat of vaporization, solubility) - ANSWERS 1) high specific heat, or heat required to raise the temperature of the unit mass of a given substance by one degree. For water to increase in temperature, water molecules must be made to move faster, or get higher KE, and doing this requires breaking hydrogen bonds, which absorbs heat. So, as heat is applied, most of it goes to breaking the bonds not upregulating KE, thus making water harder to heat than sub stances where no bonds need to be broken. 2) High heat of vaporization, or the amount of heat needed to turn one g of a liquid into vapor, without a temperature rise in the liquid. Important for sweat because it ensures that when the liquid evaporates fr om our skin, the heat required for the transition is kept in the gas, causing a net cooling effect on the skin. 3) Unique solubility properties: "like dissolves like". Water dissolves polar molecules and ions, and can act as an H -bond donor or receptor 4) Amphoteric, it can act as an acid (donating electrons) or a base (accepting electrons). The conjugate acid of water is the hydronium ion, H3O+, and the conjugate base of water is the hydroxide ion, OH -. Keq for water at 25 degrees C and in pure water - ANSWERS At 25 degrees C: Keq= Kw= [OH -][H3O+]= 1*10^ -14 In pure water: [OH-]=[H3O+]= 1*10^ -7 Calculation for pH and pKa - ANSWERS pH= -log[H3O+] pKa= -log(Ka) Normal blood pH range - ANSWERS 7.35 -7.45 The Hydrophobic Effect - ANSWERS When non -polar molecules aggregate in the presence of water, minimizing the entropy decrease water must go through to order themselves around the border of the non -polar molecule. Reducing the surface area water must organize around incr eases entropy, which is favorable. The aggregation is responsible for the formation of a variety of lipid structures in the body, including cell membranes. Buffers - ANSWERS Composed of a weak acid (HA) and its conjugate base (A -). Added acid r eacts with A -, and added base reacts with HA, giving a limited overall pH change. Two main reactions: 1) When excess base is added: OH-+HA-->H2O+A - 2) When excess acid is added: H+ + A - -->HA **So, the net result is more of the weak acid and its conjugate base** When are buffers optimal? What equation can we use for this? - ANSWERS When [HA]= [A -], occurring when pH=pKa Henderson - Hasselbalch allows use to calculate pH at given pKa, and vice versa: Blood Buffering - ANSWERS Components: 1) carbonic acid (H2CO3) (weak acid). pKa= 6.1. 2) Bicarbonate Ion (HCO3 -), conjugate base of carbonic acid 3) H+ (hydrogen ion) If OH - (base) is added, Carbonic acid buffers it into bicarb onate ion and water. If H+ (acid) is added, bicarbonate ions and H+ buffer it to carbonic acid. Amino Acids, peptides, and polypeptides - ANSWERS the building blocks of proteins, a chain of which is called a peptide. There are 20 standard amino acids that act as the monomers to make protein polymers! A long peptide is called a polypeptide! Proteins are composed of one or more polypeptide chain. Peptide bonds - ANSWERS Between the C and N of C=O and N -H of two adjacent amino acids. What wavelength is indicative of aromatic amino acids? - ANSWERS 280 nm, with tryptophan absorbing more, tyrosine absorbing a bit less, and phenylalanine absorbing a lot less. Stereochemistry of amino acids - ANSWERS Every carbon except for glycine is a chiral center, giving two possible structures for each: L and D (except for glycine). L is the only one found in nature. Acid-Base properties of amino acids - ANSWERS Each has at least two ionizable protons (from the COOH and N H3 groups), but most have others. COOH pKa: 2.34 NH3 pKa: 29.60 PI - ANSWERS the isoelectric point, or the pH at which an amino acid or peptide has no net charge. - At pH= PI, the predominant species is the zwitterion - At pH<PI, the predom inant species is net positive - At pH>PI, the predominant species is net negative **At PI, amino acid or peptide cannot migrate through an electric field, so this is a way we can separate amino acids (by PI via electric field!)** How to calculate PI, or isoelectric pint - ANSWERS Average pKa values involving the neutral species For glycine, that only has COOH and NH3 pKa's, the PI is the average between 2.34 and 9.60, so 5.97! Essential Amino Acids - ANSWERS Those that cannot be mad e in the body and thus must be obtained via the diet. Some can be made from others, so it isn't a hard line, but: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine are all essential. **Cysteine can be made from methionine, tyrosine can be made from phenylalanine, etc.** Amino acid name post -peptide bond - ANSWERS residues Peptide directionality - ANSWERS From N to C Protein makeup - ANSWERS One or more pol ypeptide chains, and optional prosthetic groups that make a conjugated protein. The folding of the chains is what delineates function of the protein itself. 4 protein structure levels, and their components - ANSWERS Primary structure: amino aci d reside sequences Secondary structure: alpha helix and beta sheet folded segments Tertiary structure: polypeptide chains in 3 -D shapes Quaternary structure: Assembled subunits comprised of multiple chains
