Proteins

sulfur used in amino acids

• formation of disulfide bonds

• covalent links between two cysteine amino acids

• crucial for stability in tertiary and quaternary structure

primary structure

the number and sequence of amino acids in a polypeptide, determined by the base sequence of the gene that codes for the polypeptide

secondary structure

held together by hydrogen bonds between oxygen of carbonyl groups (C=O) and hydrogen atoms of an amine group in the polypeptide backbone

• alpha helix

• beta pleated sheet

alpha helix

right-handed helix in which hydrogen bonds can form adjacent turns of the helix

beta pleated sheet

amino acid chains connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet

tertiary structure

folding of a single protein/polypeptide chain into a 3D structure with a hydrophobic core, giving proteins their functional properties (ex. active sites on enzymes), stabilized by:

• interactions between R-groups of amino acids

• ionic bonds

• hydrogen bonds

• hydrophobic bonds

• disulfide bridges

ionic bonds in tertiary structure

can form between positively and negatively charged R-groups

hydrogen bonds in tertiary structure

can form between some polar r groups

hydrophobic bonds in tertiary structure

weak chemical interactions that can form between non-polar R-groups

disulfide bridges in tertiary structure

strong bonds can form between pairs of cysteine

examples of tertiary structure

channel proteins and integral proteins

channel proteins

• inside lined by polar amino acids

• held in a transmembrane position by hydrophobic amino acids

• allow hydrophilic molecules like water or ions to pass through with ease

integral proteins

• polar amino acids on the outside extending into the extracellular fluid, associating with polar water molecules

• non-polar amino acids anchor the protein into the membrane, associating with hydrophobic parts of the membrane

quaternary structure of proteins

how different polypeptides are arranged together (contains more than one polypeptide) by the same types of interactions contributing to tertiary structure (hydrogen bonding, London dispersion forces)

conjugated proteins

one or more non-polypeptide subunits in addition to their polypeptides

hemoglobin as an example of a conjugated protein

• oxygen carrying protein consisting of four polypeptides

• each subunit contains one iron atom/heme (prosthetic group) and can carry one molecule of oxygen

non-conjugated proteins

structure composed only of polypeptides

• ex. insulin, collagen

fibrous proteins

tube-like, structural protein (ex. keratin, collagen)

globular proteins

round, traveling, or transmembrane proteins (ex. hemoglobin, insulin)