Kaplan MCAT Biology Ch. 15: Amino Acids, Peptides and Proteins

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Amino Acids

-an amine group and a carboxyl group to a single carbon atom (the alpha carbon)

-alpha hydrogen and R group

-alpha carbon, chiral (stereogenic) center --> optically active

-naturally occurring (20) are all L-enantiomers, amino group on the left

-S config (except for cysteine, R, priority by sulfur)

R group

attached to alpha carbon

glycine

simplest AA, not chiral

acid-base characteristic

-basic amino group (pos when protonated) and acidic carboxyl group (neg charge when depro) --> amphoteric

-depend on terms and conditions

-pH --> lots of protons -->AA as base (vice versa)

-try to achieve equilibrium

-since two diff locations, can either be pro or depro, having least two diff disso. constants Ka1 and Ka2, to pH, or Kb1 and Kb2, pOH

-neutral AA in acidic sol --> fully protonated --> amino pro easily than carboxyl (need fairly acidic)
-if even basic solution, become fully depro, easilly for carboxyl group than amino group.

amphoteric

-species acting both acids and bases (ex: h20)

-

zwitterion

both charged on amino and carboxyl at same time --> neutralize --> so at neutral pH --> internal salts

isoelectric point (pI)/isoelectric pH

-intermediate pH, which AA exists as zwitterion (uncharged)

-between pKa1 and pKa2 (pKa is pH which disasso. occurs)

titration of amino acids

-looks like a combo of two or three monoprotic acids (three is acidic or basic R group)

-when add base, carboxyl group deprotonates first, than amino group

-two moles of base to deprotonate one mole of most amino acids

-buffering capacity greatest at or near pH of two diss. constants, pKa1 and pKa2; at isoelectric point, capacity is min., vertical line

-some have acidic or basic side chains, to find pI, avg two acidic pKa;s if side chain acidic, two basic pKa's if basic

-can perform by adding acid to base, sequence is reversed.

henderson-hasselbalch equation

-relationship by relating the pH to the ratio of CA to CB

-when pKa known, ratio at particular pH can be determined

-can prepare effective buffer solutions of AAs; buffering regions of AA within one pH unit of pKa or pKb.

amino acid side chains

-side chains (R groups) give character and give proteins distinguishing features

-

types of amino acid side chains

nonpolar, polar (uncharged), acidic, basic

nonpolar amino acids

-most R groups are saturated hydrocarbons --> hydrophobic -->dec the solubility in water --> prefer bured inside proteins away from aq. cell enviwronment

-tryptophan has N atom w/ lone pair, resonated through aromatic ring --> doesn't exhibit basic properties --> nucleating residue when proteins fold

-often found at core of globular proteins or transmembrane regions of proteins in contact w/ hydrophobic portion of phospholipid membrane

polar amino acids

-uncharged polar R- groups that are hydrophilic --> inc. solubity in water --> surface of proteins

acidic amino acids

-R-group has carboxyl group, neg charge at physiological pH (7.4) so exist in salt form in body

-roles in substrate-binding site of enzymes, require proton transfer

-name ends in -acid

-have three distinct pKa's; has three groups (two COOH (overlap) and one NH3+), because of add. carboxyl group, isoelectric point shifted towards acidic pH, founding by avg. both acidic pKa's

-3 moles of base needed for deprotonation

basic amino acids

-side chain is amino group

-net pos charge at pH 7.4

-add. amino group, three disso. constant (amino's overlap)

-pI towards alkaline pH, avg two basic pKas

-three moles of acid

Predicting AA charge

using pI:

-pH < pI --> pos
-pH > pI --> neg

peptides

-amino acid subunits , sometimes called residues

-carboxyl one end, amino at other --> combine is peptide bond

-small proteins, < 50 residues

residues

amino acid units for peptides

peptide bond

two amino acids combine --> amine bond forms between them

dipeptide

two amino acids joined

tripeptide

three amino acids joined

polypeptide

many amino acids

rxns, forming peptide bonds

-condensation rxn occurs (water is lost)

-reverse, hydrolysis (cleavage by adding water), catalyzed by an acid or base

-certain enzymes digest specific peptide linkage (given in passage)

properties of peptides

-terminal amino acids
1. free alpha amino group - amino terminal, N terminal
2. free carboxyl group - carboxy-terminal, C terminal
read from N to C (left to right)

-amides have two resonance structures, w/ partial DB character between N and C --> C-N bond restrict (rigid and stable) of backbone of proteins

amino terminal, N terminal

free alpha amino group of terminal amino acid

carboxy-terminal, C terminal

free carboxyl group of terminal amino acid

proteins

-polypeptides

-range in length

-many functions
1. hormones
2. enzymes
3. membrane pores
4. receptors
5. elements of cell structures
6. main actors of bio system

-ther are four levels

primary structure of protein

-structure coded in DNA of organism

-sequence of AA, from N-term to C-term, linked by peptide bonds

-most fund. structure, seq. determines higher levels of protein structure (2,3,4 most energetically favorable)

-determined in lab through sequencing, easily done on DNA that produced protein

sequencing

1st structure determined in lab this way, easily done on DNA that produced protein

secondary structure

-local structure of neighboring AAs

-result of H bonding between AAs

-two most common types are alpha helix and beta pleated sheet

alpha helix

-rodlike structure, peptide chain coils clockwise about central axis

-helix stablized by intramolecular H bonds between carbonyl oxygen atoms and amide H atoms foru residues away from each other (n +4 H bond)

-side chains point away from helix core, interacting w/ cell environment

-typical protein w/ structure is keratin

keratin

fibrous structural protein; hair and fingernails

B pleated sheet

-may be parallel or antiparallel

-peptide chains lie alongside each other, forming rows

-chains held by intramolecular H bonds, between carbonyl O atoms on one peptide chain and amine H atom on another

-rippled, or pleated, shape

-R-groups of amino residues point above and below plane

-ex: silk fibers

tertiary structure

-3D shape

-determined by hydrophobic and hydrophilic between R groups of amino acids

-also determined by distribution of disulfide bonds (create loops)

-proline --> ring --> can't fit every location of alpha helix --> kink in chain

-two major classifications: fibrous proteins and globular proteins

cysteine

disulfide bonds results when these two moleculse become oxidized to from cystine

fibrous proteins

like collagen, as sheets or long strands

globular proteins

like myoglobin, are spherical

two major classifications of tertiary structures

fibrous and globular proteins

quarternary structure

-more than one polypeptide subunit

-refers way these subunits arrange themselves yield functional protein

-ex: hemoglobin

hemoglobin

-O2-transporting machines --> fill red bloodcells

-composed of 4 diff. globular protein subunits

Conjugated Proteins

-part of function from covalently attached molecules called prosthetic groups

-proteins w/ lipid, carb, and nucleic acid prosthetic groups: lipoproteins, glycoproteins, nucleoproteins.

-major roles in determining function of their respective proteins

-Hemoglobin's subunit (also myoglobin) have heme group

prosthetic roups

-conjugated proteins get part of function from these covalently attached molecules.

-can be organic (vitamins) or even metals ions

heme group

-prosthetic group of hemoglobin (cooperative) (and myoglobin subunits
(inactive w/o)
-composed of organic porphyrin ring w/ iron atom in center

-binds to and carrier oxygen

denaturation of proteins

-also known as melting

-protein loses 3D structure and revert to a random-coil state

random-coil

-state achieved after protein loses 3D structure

-completely functionless, damage usually permanent; gentle denaturing agent (urea) don't permanently

-methods: detergent, change in pH, temp, or even solute concentration; removing reagent might renature (regain structure and function)

-weak intermolecular forces --> protein stable and function --> disrupted

-reversible

renature

removing reagent that denatures protein brings to this state

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