Lecture 7 8 - final review.docx

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Biochem & Molecular Biology
BIOC 2300
Dr.Carmichael Wallace

Lecture 7/8 Proteins are distinguished by the number/composition/sequence of amino acid residues - amino acid polymers <50 amino acids = peptides - >50 amino acids = proteins/polypeptides - There are 20 standard amino acids o 19 have the same general structure – central alpha carbon, an amino group, a carboxylate group, hydrogen atom, an R group (exception is proline)  R group gives the amino acid its unique properties  Amino acids are classified by their ability to interact with water o At pH 7, the carboxyl group is in conjugate base form, the amino group is in conjugate acid form therefore amphoteric (molecule that can act as both an acid and a base) o Zwitterions = molecules that have both positive and negative charges on different atoms and therefore have no net charge at pH 7 Amino Acid Classes - Polar amino acids have functional groups that can easily interact with water through hydrogen bonding o Contain a hydroxyl group  Serine  Threonine  Tyrosine o Or contain an amide group  Asparagine - Acidic amino acids have side chains with a carboxylate group that ionizes at physiological pH o Aspartate o Glutamate - Basic amino acids have a positive charge at physiological pH o At physiological pH lysine is its conjugate acid (-NH3+), arginine is permanently pronated, and histidine is a weak base (only partly ionized) Amino Acids have various Biological Roles 1) Some amino acids/derivatives can act as chemical messengers – neurotransmitters (tryptophan-derivative serotonin) and hormones (tyrosine-derivative thyroxine) 2) Act as precursors for other molecules (nucleotides and heme) 3) Metabolic intermediates (arginine, ornithine, and citrulline in the urea cycle) Modified Amino Acids in Proteins - Some proteins have amino acids that can be modified after synthesis o Serine, threonine, tyrosine  can be phosphorylated - Amino acid stereoisomers  because the alpha/chiral carbon is attached to four different groups, they can exist as stereoisomers o EXCEPT GLYCINE (nonchiral standard amino acid) o The molecules are mirror images of one another – enantiomers  Cannot be superimposed over one another  Rotate plane, polarized light in opposite directions – optical isomers o Molecules are designated D or L (used to indicate the similarity of the arrangement of atoms around the molecule’s asymmetric carbon to the asymmetric carbon of the glyceraldehyde isomer – reference compound for optical isomers) o Chirality is important for the structure and function of proteins Titration of Amino Acids - Free amino acids contain ionizable groups – ionic form depends on the pH - Titration of an amino acid
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