Amino acids

Amino Acid Monomers Amino acids are organic molecules with carboxyl and amino groups Amino acids are the building blocks of proteins. Contain a carboxylic acid group and an amino group on the alpha () carbon. Amino acids differ in their properties due to differing side chains, called R groups

Amino group Carboxyl group -Carbon Amino group Carboxyl group

20 amino acids make up protein Amino Acid Monomers 20 amino acids make up protein Dipolar (Have both charges) +ve end (NH3+) -ve end (COO-)

The 20 amino acids of proteins Nonpolar Glycine (Gly or G) Alanine (Ala or A) Valine (Val or V) Leucine (Leu or L) Isoleucine (Ile or I) Methionine (Met or M) Phenylalanine (Phe or F) Trypotphan (Trp or W) Proline (Pro or P) Polar Serine (Ser or S) Threonine (Thr or T) Cysteine (Cys or C) Tyrosine (Tyr or Y) Asparagine (Asn or N) Glutamine (Gln or Q) Electrically charged Acidic Basic Aspartic acid (Asp or D) Glutamic acid (Glu or E) Lysine (Lys or K) Arginine (Arg or R) Histidine (His or H) The 20 amino acids of proteins

Nonpolar Glycine (Gly or G) Alanine (Ala or A) Valine (Val or V) Leucine (Leu or L) Isoleucine (Ile or I) Figure 5.17 The 20 amino acids of proteins Methionine (Met or M) Phenylalanine (Phe or F) Tryptophan (Trp or W) Proline (Pro or P)

Polar Asparagine (Asn or N) Glutamine (Gln or Q) Serine (Ser or S) Threonine (Thr or T) Cysteine (Cys or C) Tyrosine (Tyr or Y) Figure 5.17 The 20 amino acids of proteins

Electrically charged Acidic Basic Aspartic acid (Asp or D) Figure 5.17 The 20 amino acids of proteins Aspartic acid (Asp or D) Glutamic acid (Glu or E) Lysine (Lys or K) Arginine (Arg or R) Histidine (His or H)

Essential Amino Acids Group of amino acids not synthesized by the body arg, his, ile, leu, lys, met, phe, thr, trp, val Must obtain from the diet All in diary products 1 or more missing in grains and vegetables

Complete protein Proteins in diet comes from both animal and vegetable sources. Most animal sources (meat, milk, eggs) provide complete protein (They contain all of the essential amino acids. Vegetable sources usually low on or missing some essential amino acids.

Amino acids stereoisomers All amino acids except glycine are chiral. Amino acids have stereoisomers. In biological systems, only L-amino acids are used in proteins. L-Alanine D-Alanine L-Cysteine D-Cysteine

Zwitter ion Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water. Zwitter ion = double ion – when both NH3+ and COO- are ionized COO- can accept a proton, NH3+ can accept electron, so can act as acid and base (amphoteric) At the isoelectric point (pI), the + and – charges in the zwitterion are equal.

Amino Acids as Acids In solutions more basic than the pI, the —NH3+ in the amino acid donates a proton. + OH– H3N—CH2—COO– H2N—CH2—COO– Zwitter ion at pI Negative ion

Amino Acids as Bases In solution more acidic than the pI, the COO- in the amino acid accepts a proton. + H+ + H3N—CH2—COO– H3N—CH2—COOH Zwitterion at pI Positive ion

pH and Ionization Acidic amino acids such as aspartic acid have a second carboxyl group that can donate and accept protons. The pI for aspartic acid occurs at a pH of 2.8.

Amino Acid Polymers Amino acids are linked by peptide bonds A polypeptide is a polymer of amino acids Polypeptides range in length from a few to more than a thousand monomers Each polypeptide has a unique linear sequence of amino acids

Making a polypeptide chain bond Amino end (N-terminus) Side chains Backbone Carboxyl end (C-terminus) (a) (b) Making a polypeptide chain Figure 5.18

If only one a.a. begins with a certain letter, that letter is used Cysteine = Cys = C Histidine = His = H Isoleucine = Ile = I Methionine = Met = M Serine = Ser = S Valine = Val = V

If more than one a.a. begins with a certain letter, that letter is assigned to the most commonly occurring one Alanine = Ala = A Glycine = Gly = G Leucine = Leu = L Proline = Pro = P Threonine = Thr = T

Phonetically suggestive Aspartic acid (“asparDic acid”) = Asp = D Phenylalanine (“Fenylalanine”) = Phe = F Arginine (“aRginine”) = Arg = R Tyrosine (“tYrosine”) = Tyr = Y Tryptophan (double ring in the molecule) = Trp = W

A letter close to the initial is used Asparagine (contains N) = Asn = N Glutamic acid (near D) = Glu = E Glutamine (“Q-tamine”) = Gln = Q Lysine (near L) = Lys = K