Transcript of amino acids

Chapter 5
The Structure and Function of
Macromolecules
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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• Overview: The Molecules of Life
– Another level in the hierarchy of biological
organization is reached when small organic
molecules are joined together
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• Macromolecules
– Are large molecules composed of smaller
molecules
– Are complex in their structures
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Concept 5.1: Most macromolecules are
polymers, built from monomers
• Three of the classes of life’s organic
molecules are polymers
– Carbohydrates
– Proteins
– Nucleic acids
• The fourth class is not a polymer (the lipids)
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• A polymer (poly=many; mer=part)
– Is a long molecule consisting of many
similar building blocks called monomers
(mono=single)
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The Synthesis and Breakdown of Polymers
• Monomers form larger molecules by
condensation reactions called dehydration
(polymerization) reactions
• Requires energy
• Requires enzymes
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• Polymers can disassemble by
– Hydrolysis: (hydro= water; lysis= break)
• Releases energy
• Enzymes speed up hydrolysis
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The Diversity of Polymers
• Each class of polymer
– Is formed from a specific set of monomers
1
2
3
H
HO
• Although organisms share the same limited
number of monomer types, each organism is
unique based on the arrangement of
monomers into polymers
• An immense variety of polymers can be built
from a small set of monomers
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•
Concept 5.2: Carbohydrates serve as fuel
and building material
•
Carbohydrates
–
Include both sugars and their polymers
–
Monomers of carbohydrates are simple
sugars called Monosaccharides
–
Polymers are formed by condensation
reaction
–
Are classified based on the number of
simple sugars
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Sugars
•
Monosaccharides
–
Mono=single, sacchar=sugar
–
Are the simple sugars in which C, H and O
are occur in the ratio of CH2O.
–
Are major nutrients for the cell
–
Can be produced by photosynthesis from
CO2, H2O and sunlight.
–
Store energy in their chemical bonds
which are harvested by cellular
respiration.
–
Can be incorporated as monomers into
disaccharides and polysaccharides
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• Monosaccharides
– May be linear
– Can form rings
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• Disaccharides
– (Di=two; sacchar=sugar)
– consists of two monosaccharides joined by
glycosidic linkage
– Maltose (malt sugar) = glucose + glucose
– Lactose (milk sugar) = glucose + galactose
– Sucrose (table sugar) = glucose + fructose
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Polysaccharides
•
Polysaccharides
–
Macromolecules that are polymers of a
few hundred or thousand of
monosaccharides.
–
Formed by linking monomers in
condensation reaction
–
Have two important biological functions:
i. energy storage (starch and glycogen)
ii. structural support (cellulose and chitin).
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Storage Polysaccharides
•
Starch
–
Is a polymer consisting entirely of
glucose monomers
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Starch
– Is the major storage
form of glucose in
plants
– Stored as granules
within plant
organelles called
plastids
– Amylose the
simplest form is an
unbranched polymer.
– Amylopectin is
branched polymer
– Most animals have
digestive enzymes to
hydrolyse starch
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Glycogen
Large glucose polymer that is more highly
branched than amylopectin
Is the major storage form of
glucose in animals
Stored in the muscles and
liver of humans and other
vertebrates
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Structural Polysaccharides
•
Cellulose
–
Linear unbranched polymer of glucose
–
Differ from starch in its glycosidic
linkages
–
Cellulose and starch have different threedimensional shapes and properties as a
result of differences in glycosidic
linkages.
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– Has different glycosidic linkages than
starch
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– Is a major component of the tough walls
that enclose plant cells
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• Cellulose is difficult to digest
– Cows have microbes in their stomachs to
facilitate this process
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• Chitin, another important structural
polysaccharide
– Is found in the exoskeleton of arthropods
– Can be used as surgical thread
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• Concept 5.3: Lipids are a diverse group of
hydrophobic molecules
• Lipids
– Are the one class of large biological
molecules that do not consist of polymers
– Share the common trait of being
hydrophobic
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Fats
– Are constructed from two types of smaller
molecules
– a single glycerol (a three carbon alcohol)
and usually three fatty acids (carboxylic
acid)
H
Fats are formed by a
condensation reaction
which links glycerol to
fatty acids by an Ester
linkage.
Figure 5.11
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O
H
H
H
H
H
H
H
H
H
H
H
H
(b) Fat molecule (triacylglycerol)
H
H
H
•
Fatty acids
–
composed of a carboxyl group at one end
(head) and an attached hydrocarbon (C-H)
chain (tail)
–
Nonpolar C-H bonds make the chain
hydrophobic (not water soluble)
–
Vary in the length and number and
locations of double bonds they contain
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• Saturated fatty acids
– Have the maximum number of hydrogen
atoms possible
– Have no double bonds
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• Unsaturated fatty acids
– Have one or more double bonds
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Saturated fatty acids
Unsaturated fatty acids
- No double bonds
between carbons of fatty
acid tail.
- Carbon skeleton of
fatty acid is bonded to
maximum number of
hydrogens
-Usually a solid at room
temperature
-Most animal fats
- One or more double bonds
between carbons of fatty acid
tail
-Tail kinks at each C=C, so
molecules do not pack enough
to solidify at room temperature.
-Usually a liquid
temperature
- Most plant fats
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at
room
Phospholipids
• Phospholipids
– Have only two fatty acids
– Have a phosphate group instead of a third
fatty acid
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• Phospholipid structure
– Consists of a hydrophilic “head” and
hydrophobic “tails” → it’s amphiphatic
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• The structure of phospholipids
– Results in a bilayer arrangement found in
cell membranes
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Steroids
• Steroids
– Are lipids characterized by a carbon
skeleton consisting of four fused rings
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• One steroid, cholesterol
– Is found in cell membranes
– Is a precursor for some hormones
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• Concept 5.4: Proteins have many structures,
resulting in a wide range of functions
– Proteins
• Have many roles inside the cell
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• An overview of protein functions
- Are abundant, forming about 50% of cellular dry weight
- Have important functions in the cell:
1.structural support
2.storage (of amino acids)
3.transport (e.g. hemoglobin)
4. signaling (chemical messengers)
5.cellular response (receptor proteins)
6.movement (contractile proteins)
7.defense (antibodies)
8.catalysts of biochemical reactions (enzymes
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• Enzymes
– Are a type of protein that acts as a catalyst,
speeding up chemical reactions
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Polypeptides
• Polypeptides
– Are polymers of amino acids
• A protein
– Consists of one or more polypeptides
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Amino Acid Monomers
• Amino acids
– Are organic molecules possessing both
carboxyl and amino groups
– Differ in their properties due to differing
side chains, called R groups
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• 20 different amino acids make up proteins
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Amino Acid Polymers
• Amino acids
– Are linked by peptide bonds
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Determining the Amino Acid Sequence of a Polypeptide
• The amino acid sequences of polypeptides
– Were first determined using chemical
means
– Can now be determined by automated
machines
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Protein Conformation and Function
• A protein’s specific conformation
– Determines how it functions
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• Two models of protein conformation
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Four Levels of Protein Structure
•
Primary structure
–
Is the unique
sequence of amino
acids in a polypeptide
–
determined by genes
–
slight change can
effect the protein
conformation and
function (e.g. sicklecell hemoglobin)
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• Secondary structure
–
Is the folding or coiling of the polypeptide into a repeating
configuration
–
Includes the  helix and the  pleated sheet
–
Stabilized by hydrogen bonding
H
H
Figure 5.20
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• Tertiary structure
– Is the overall three-dimensional shape of a
polypeptide
– Results from interactions between amino
acids and R groups
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• Quaternary structure
– Is the overall protein structure that results
from the aggregation of two or more
polypeptide subunits
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• The four levels of protein structure
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What Determines Protein Conformation?
•
Protein conformation
–
A proteins three-dimensional shape is a
consequence of the interactions
responsible for the secondary and tertiary
structures.
–
This conformation is influenced by
physical & chemical environmental
conditions.
–
If a protein’s environment is changed, it
may become denatured and lose its
conformation.
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•
Denaturation
–
Is when a protein unravels and loses its native conformation
–
A protein can be denatured by:
•
transfer to organic solution.
•
Chemical agent that disrupt hydrogen bonds.
•
Excessive heat that disrupt weak interactions
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• Concept 5.5: Nucleic acids store and
transmit hereditary information
• Genes
– Are the units of inheritance
– Program the amino acid sequence of
polypeptides
– Are made of nucleic acids
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The Roles of Nucleic Acids
• There are two types of nucleic acids
– Deoxyribonucleic acid (DNA)
– Ribonucleic acid (RNA)
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•
DNA
–
Stores information for the synthesis of specific
proteins
–
contains genes that program all cell activity.
–
Contain directions for its own replication
–
Is copied and passed from one generation to
another.
–
In eukaryotic cells, is found in the nucleus.
–
Makes up genes that contain instructions for
protein synthesis.
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– Directs RNA synthesis
– Directs protein synthesis through RNA
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RNA
•
function in the actual synthesis of proteins
•
Sites of protein synthesis are on ribosomes
in the cytoplasm.
•
Messenger RNA (mRNA) carries encoded
message from the nucleus to the cytoplasm
•
The flow of genetic information goes from
DNA  RNA protein (central dogma)
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The Structure of Nucleic Acids
• Nucleic acids
– Exist as polymers called polynucleotides
(a) Polynucleotide,
or nucleic acid
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• Each polynucleotide
– Consists of monomers called nucleotides
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Nucleotide Monomers
Are made up of nucleosides and
phosphate groups
Purine: Characterized by a fivemembered ring fused to a sixmembered ring.
Examples
- Adenine (A)
- Guanine (G)
Pyrimidine: Characterized
by a six-membered ring
made up of carbon and
nitrogen atoms.
Examples:
- Cytosine (C)
- Thymine (T); found
only in DNA
- Uracile (U); found only
in RNA
Figure 5.26
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(c) Nucleoside components
Nucleotide Polymers
• Nucleotide polymers
– Are made up of nucleotides linked by the
–OH group on the 3´ carbon of one nucleotide
and the phosphate on the 5´ carbon on the
next → phosphodiester linkages
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• The sequence of bases along a nucleotide
polymer
– Is unique for each gene
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The DNA Double Helix
• Cellular DNA molecules
– Have two polynucleotides that spiral
around an imaginary axis
– Form a double helix
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• The DNA double helix
– Consists of two antiparallel nucleotide
strands
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• The nitrogenous bases in DNA
– Form hydrogen bonds in a complementary
fashion (A with T only, and C with G only)
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