Amino Acids and Proteins

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Amino Acids and Proteins

• Biology 12
• E. McIntyre

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Where do we find proteins?

• What do gelatin desserts, hair, antibodies,
  spider webs, blood clots, egg whites, tofu, and
  fingernails all have in common? They are all made
  of protein.

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Proteins

• genetic information in DNA codes specifically for
  the production of proteins
• 50 of the dry mass of most cells
• As with carbohydrates and lipids, their
  three-dimensional structure is directly related
  to their function
• enzymes are proteins

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Proteins Basic Characteristics

• Proteins are amino acid polymers
• central carbon atom surrounded by an amino group,
  a carboxyl group, a hydrogen atom, and a variable
  group of atoms called a side chain, usually
  symbolized by the letter R. Remember that
  carboxyl and amino groups are functional groups.
• There are 20 different R groups commonly found in
  living organisms and, so there are 20 different
  amino acids.

General structure of an amino acid
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• There are 20 amino acids.
• Note all are the same except for the side chains

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Amino acids General Properties

• Amino acids are amphiprotic (which means that
  they possess both acidic (carboxyl) and basic
  (amino) functional groups.)
• When dissolved in water, the carboxyl group
  donates an H ion to the amino group, causing the
  carboxyl group to become negatively charged and
  the amino group to possess an extra hydrogen and
  a net positive charge.

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Amino acids General Properties

• Amino acids may be polar (hydrophilic), nonpolar
  (hydrophobic), or charged (acidic or basic)
• Acidic amino acids possess a carboxyl group on
  their side chains, and basic amino acids contain
  amino groups on their side chains.
• These characteristics are determined by an amino
  acids side chain.

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Examples of Amino Acids

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Types of Amino Acids

• Nonpolar R H, CH3
• O
• Polar ll
• R CH2OH, CH2SH, CH2CNH2,
• (polar groups with O-, -SH, -N-)
• Acidic
• R CH2COOH, or -COOH
• Basic
• R CH2CH2NH2

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Learning Check AA1

• Identify each as polar or nonpolar
• A.
• B.

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Solution AA1

• Identify each as polar or nonpolar
• A
• B

Non-polar
polar
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Essential Amino Acids

• 10 amino acids not synthesized by the body
• Arginine (arg) histadine (his) isoleucine (ile)
• leucine (leu) threonine (thr) lysine (lys),
  methionine (met) phenylalanine (phe) trytophan
  (trp)
• valine (val)
• Must obtain from the diet
• All in diary products
• 1 or more missing in grains and vegetables
• Want to remember the essential amino acids?
• Any Help In Learning These Little Molecules
  Proves Truly Valuable

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Amino Acids as Acids and Bases

• Ionization of the NH2 (amino) and the COOH
  (carboxyl) group
• An amphiprotic molecule has both a and
  charge
• An amphiprotic molecule is neutral overall
• Zwitterion of alanine
• In water, an amino acid loses a H ion from the
  carboxyl group and
• donates it to the amino group.

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pH and ionization

• H OH
• H3NCH2COOH H3NCH2COO H2NCH2COO
• Positive ion zwitterion Negative ion
• Low pH neutral pH High pH

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Learning Check AA2

• CH3 CH3
• H3NCHCOOH H2NCH2COO
• (1) (2)
• Select from the above structures
• A. Alanine in base.
• B. Alanine in acid.

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Solution AA2

• CH3 CH3
• H3NCHCOOH H2NCH2COO
• (1) (2)
• Select from the above structures
• (2) Alanine in base.
• (1) Alanine in acid.

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Stop Think

• Remember that carboxyl and amino groups are
  functional groups.
• Think What are functional groups?
• How are polymers of amino acids made?

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Solution to Stop Think

• Carboxyl groups and amino groups react together
  to link amino acids.

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The Peptide Bond

• Amide bond formed by the COOH of an amino acid
  and the NH2 of the next amino acid
• O CH3
• 
  
• NH3CH2COH H3NCHCOO
• O CH3
• NH3CH2C NCHCOO
• peptide bond
• H

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Peptides

• Amino acids linked by amide (peptide) bonds
• Gly Lys Phe Arg
  Ser
• H2N- -COOH
• end Peptide bonds end
• Glycyllysylphenylalanylarginylserine

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Learning Check AA3

• What are the possible tripeptides formed from
  one each of leucine, glycine, and alanine?

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Solution AA3

• Tripeptides possible from one each of leucine,
  glycine, and alanine
• Leu-Gly-Ala
• Leu-Ala-Gly
• Ala-Leu-Gly
• Ala-Gly-Leu
• Gly-Ala-Leu
• Gly-Leu-Ala

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Learning Check AA4

• Write the three-letter abbreviations for the
  following tetrapeptide

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Solution AA4

• Ala-Leu-Cys-Met

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Types of Proteins

• Type Examples
• Structural tendons, cartilage, hair, nails
• Contractile muscles
• Transport hemoglobin
• Storage milk
• Hormonal insulin, growth hormone
• Enzyme catalyzes reactions in cells
• Protection immune response

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Protein Structure