Proteins Flashcards
What is the definition of protein?
- polypeptides that range in length
- function as enzymes, hormones, membrane pores, and receptors
- elements of cell structure
What is the primary structure of an amino acid? What type of bonds stabilize this structure?
- linear arrangement of amino acids coded by DNA
- stabilized by the formation of covalent peptide bonds between adjacent amino acids
- encodes for all the information needed for folding at higher structural levels
What is the secondary structure of a protein? What type of bonds stabilize this structure?
- local structure determined by nearby amino acids
- stabilized by H bonding between different amino acid residues
- include two subtypes: alpha-helices and beta-pleated sheets
What are alpha-helices?
- part of secondary protein structure
- rodlike structures in which the peptide chain coils clockwise around a central axis
- stabilized be intramolecular H bonds between carbonyl oxygen and amide H atom 4 residues down the chain
- side chains of amino acids point away from the helix core
What are beta-pleated sheets?
- part of secondary structure of proteins
- either parallel or antiparallel
- peptide chains are alongside eachother
- stabilized by intramolecular bonds between carbonyl oxygen atoms on one chain and amide H atoms in an adjacent cahin
- side chains of amino acids point above and below the plane of the sheet
What role does proline play in secondary protein structure?
- proline has a rigid cyclic structure that creates a kink in the peptide chain when in the middle of an alpha-helix
- found between chains in beta-pleated sheets
What is the tertiary structure of a protein? What bonds stabilize this structure?
- 3D shape of a single polypeptide chain
- stabilized by hydrophobic interactions, acid-base interactions (salt bridges), H-bonding, and disulfide bonds
What is a disulfide bond?
-occur when 2 cysteine molecules are oxidized and create a covalent bond to form cystine
Explain hydrophobic interactions in proteins
- these interactions push hydrophobic R groups to the interior of a protein
- leads to an increase in entropy (S) of surrounding H2O molecules
- creates a negative Gibbs free energy
What is the quaternary structure of a protein? What bonds stabilize this structure?
- not all proteins have this structure
- exist for proteins that contain more than one polypeptide chain
- Ex. hemoglobin
- structure is an aggregate of smaller globular peptides (subunits) and represents the functional form of a protein
- stabilized by hydrophobic interactions btw nonpolar side chains, electrostatic interactions btw ionic groups of opposite charge, H bonds btw polar groups, disulfide bonds
What are the functions of the quaternary protein structure?
- stability
- reduce amount of DNA needed to encode the protein complex
- bring catalytic sites closer together
- induce cooperativity
What are conjugated proteins?
proteins with covalently attached molecules called prosthetic groups – can be a metal ion, vitamin, lipid, carbohydrate, nucleic acid
Lipoproteins
have a lipid prosthetic group
Glycoproteins
have a carbohydrate prosthetic group
Nucleoproteins
have a nucleic acid prosthetic group
What is protein denaturation?
- occurs when a protein loses its 3D structure and can no longer catalyze reactions (inactive)
- heat does this by increasing the average KE thus disrupting hydrophobic interactions
- solutes do this by disrupting elements of secondary, tertiary, and quaternary structure
Structural Proteins
- have highly repetitive secondary structure and supersecondary structure – repetitive organization of secondary structural elements called a motif
- fibrous nature
- intermolecular forces maintain tertiary structure that give each protein a character shape
- includes collagen, elastin, keratin
Collagen
- comprised of 3 alpha-helical motifs
- strong and flexible
- found in connective tissue, extracellular matrix, bone, muscle, skin
Elastin
- found in extracellular matrix of connective tissue
- stretches and recoils like a spring to restore original tissue shape
- highly coiled and cross-linked
Keratin
- found in intermediate filaments in epithelial cells
- contribute to integrity of cell and function as regulatory protein
- includes actin and tubulin
Actin
- protein that makes up microfilaments and thin filaments in myofibrils (rod of muscles)
- major muscle component
- have positive and negative side so motor proteins can travel unidirectionally along filament
What is the most abundant structural protein in eukaryotes?
actin
Tubulin
- protein that makes up microtubules
- has polarity – negative end located adjacent to the nucleus and positive end in periphery of cell
- aid in cell division and intercellular transport via motor proteins
Motor Proteins
- display enzymatic activity by acting as ATPase which can power conformational change necessary for motor function
- have transport interactions with actin and/or microtubules
- includes myosin, kinesins, dyneins