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Chapter 16 Amino Acids, Proteins, and Enzymes

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Chapter 16 Amino Acids, Proteins, and Enzymes 16.1 Functions of Proteins 16.2 Amino Acids 16.3 Amino Acids as Acids and Bases Functions of Proteins Proteins perform ... – PowerPoint PPT presentation

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Title: Chapter 16 Amino Acids, Proteins, and Enzymes


1
Chapter 16 Amino Acids, Proteins, and Enzymes
  • 16.1 Functions of Proteins
  • 16.2 Amino Acids
  • 16.3 Amino Acids as Acids and Bases

2
Functions of Proteins
  • Proteins perform many different functions in the
    body.

3
Amino Acids
  • Amino acids
  • are the building blocks of proteins.
  • contain a carboxylic acid group and an amino
    group on the alpha (?) carbon.
  • are ionized in solution.
  • each contain a different side group (R).
  • R side chain R
  • H2NC COOH H3NC COO-
  • H H ionized form

4
Examples of Amino Acids
  • H
  • H3NCCOO-
  • H glycine
  • CH3
  • H3NCCOO-
  • H alanine

5
Types of Amino Acids
  • Amino acids are classified as
  • nonpolar (hydrophobic) with hydrocarbon side
    chains.
  • polar (hydrophilic) with polar or ionic side
    chains.
  • acidic (hydrophilic) with acidic side chains.
  • basic (hydrophilic) with NH2 side chains.

Nonpolar Polar
Acidic
Basic
6
Nonpolar Amino Acids
  • An amino acid is nonpolar when the R group is H,
    alkyl, or aromatic.

7
Polar Amino Acids
  • An amino acid is polar when the R group is an
    alcohol, thiol, or amide.

8
Acidic and Basic Amino Acids
  • An amino acid is
  • acidic when the R group is a carboxylic acid.
  • basic when the R group is an amine.

9
Learning Check
  • Identify each as (1) polar or (2) nonpolar.
  • A. H3NCH2COO- (Glycine)
  • CH3
  • CHOH
  • B. H3NCHCOO - (Threonine)

10
Solution
  • Identify each as (1) polar or (2) nonpolar.
  • A. H3NCH2COO- (Glycine) (2) nonpolar
  • CH3
  • CHOH
  • B. H3NCHCOO - (Threonine) (1) polar

11
Fischer Projections of Amino Acids
  • Amino acids
  • are chiral except glycine.
  • have Fischer projections that are stereoisomers.
  • that are L are the only amino acids used in
    proteins.
  • L-Alanine D-Alanine
    L-Cysteine D-Cysteine

12
Zwitterions
  • A zwitterion
  • has charged -NH3 and COO groups.
  • forms when both the NH2 and the COOH groups in
    an amino acid ionize in water.
  • has equal and charges at the isoelectric
    point (pI).
  • O
    O
  • NH2CH2COH H3NCH2CO
  • glycine zwitterion
    of glycine

13
Amino Acids as Acids
  • In solutions more basic than the pI,
  • the NH3 in the amino acid donates a proton.
  • OH
  • H3NCH2COO H2NCH2COO
  • zwitterion Negative ion
  • at pI pH gt pI
  • Charge 0 Charge 1-

14
Amino Acids as Bases
  • In solution more acidic than the pI,
  • the COO- in the amino acid accepts a proton.
  • H
  • H3NCH2COO H3NCH2COOH
  • zwitterion Positive ion
  • at pI pHlt pI
  • Charge 0 Charge 1

15
pH and ionization
  • H OH
  • H3NCH2COOH H3NCH2COO H2NCH2COO
  • positive ion zwitterion
    negative ion
  • low pH pI high pH

16
Separation of Amino Acids
  • When an electric current is used to separate a
    mixture of
  • amino acids
  • the positively charged amino acids move towards
    the negative electrode.
  • the negatively charged amino acids move toward
    the positive electrode.
  • an amino acid at its pI does not migrate.
  • the amino acids are identified as separate bands
    on the filter paper or thin layer plate.

17
Separation of Amino Acids
  • With an electric current, a mixture of lysine,
    aspartate,
  • and valine are separated.

18
Learning Check
  • CH3 CH3
  • H3NCHCOOH H2NCHCOO
  • (1) (2)
  • Which structure represents
  • A. Alanine at a pH above its pI?
  • B. Alanine at a pH below its pI?

19
Solution
  • CH3 CH3
  • H3NCHCOOH H2NCHCOO
  • (1) (2)
  • Which structure represents
  • A. Alanine at a pH above its pI? (2)
  • B. Alanine at a pH below its pI? (1)
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