Amino Acids, Proteins & Enzymes Chapter 16 DE CHEMISTRY – King William HS Amino Acids, Proteins & Enzymes Chapter 16

Amino Acids Building blocks of proteins Central carbon atom bonded to two functional groups (ammonium & carboxylate) called the a carbon The a carbon is also bonded to hydrogen and a side chain Different side chains = different amino acids

Amino acid structure

Different amino acids

Classification of amino acids Nonpolar  R = hydrogen, alkyl or aromatic group = hydrophobic Polar  R = hydroxyl, thiol or amide group = hydrophilic Acidic  R = carboxylate group Basic  R = amino group which ionized to give ammonium

Amino acid stereoisomers All a carbon are attached to four different atoms (EXCEPT glycine)…so they are chiral! Amino acids can be labeled as D and L stereoisomers

Amino acids as acids and bases Isoelectric point = pH where amino acid is neutral (usually between 5.1-6.3)

Proteins Linked amino acids through peptide bonds Peptide bond – an amide bond that forms when the –COO- group of one amino acid reacts with the –NH3+ of another amino acid Dipeptide = two amino acids Tripeptide = three amino acids Polypeptide = many amino acids N-terminal amino acid (far left amino acid and has a free NH3+ group

Peptide bonds C-terminal end amino acid (far right and has a free COO- group

Naming Peptides

Primary structure of the protein Protein – polypeptide of 50 or more amino acids Primary structure – sequence of amino acids held together

Secondary structure of proteins Secondary – the geometry (shape) of the chain (helix or sheet) Alpha helix looks like a spiral staircase Beta pleated sheet look like sheets of paper (les common than alpha helix) Triple helix has three peptide chains woven together (EX: Collagen – the most abundant protein in the body – about 1/3 of proteins in vertebrates)

Alpha (a) Helix

Beta (b) Pleated Sheets

Triple helix

Tertiary Structure Attractions and repulsions between R groups of amino acids in the polypeptide chain 1. Hydrophobic interactions 2. Hydrophilic interactions 3. Salt bridge 4. Hydrogen bonding 5. Disulfide bonds

Tertiary Structures

Globular Proteins Globular – spherical in shape because polypeptide chains fold over on top of each other because of R groups interactions (tertiary structure) 1. cell synthesis, transport & metabolism EX: myoglobin (stores O2 in skeletal muscles) 1. high [ ] in sea mammals (can stay under water longer

Fibrous Proteins Long, thin fiber shaped proteins usually involved in cell/tissue structure 1. alpha keratins (hair, wool, skin & nails)  alpha helix 2. beta keratins (feathers of birds & scales or reptiles)  beta pleated sheets

Quaternary Structure Contains two or more tertiary subunits (protein chains) Held together by the same interactions as tertiary structures EX: Hemoglobin

Hemoglobin Has four subunits (a2b2) and they must all be combined to transport oxygen (4 molecules)

Denaturation of Proteins Disruption/destruction of secondary and tertiary structures of a protein 1. increase in temperature 2. change in pH No longer biologically active Proteins unfold

Enzymes Biological catalysts (increases the rate of reaction by lowering energy of activation) Enzyme names end with “ase” Most are globular proteins

Types of Enzymes

Factors affecting enzyme activity Temperature (directly proportional) 1. optimal temperature (37oC) 2. > 50oC (protein shape destroyed… therefore enzyme activity = zero) pH 1. optimal pH (7.4) 2. changes destroy structure and enzyme no longer binds to substrate

Enzyme inhibition Inhibitors cause enzymes to lose catalytic ability Competitive inhibitors – structure and polarity similar to substrate Noncompetitive inhibitors – structure does not resemble the substrate

Competitive inhibitors

Noncompetitive inhibitors

Irreversible inhibition Enzyme permanently loses all catalytic capability