proteins and nucleic acids · the roles of nucleic acids there are two types of nucleic...
TRANSCRIPT
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NOTES: Ch 5,
part 2 -
Proteins &
Nucleic Acids
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5.4 - Proteins have many
structures, resulting in a wide
range of functions
● Proteins account for more than 50% of the dry
mass of most cells
● Protein functions include:
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Functions of Proteins
● structural support
● storage
● cell-to-cell communication / signaling
● movement
● transport
● response
● defense
● catalysis of reactions (enzymes)
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● an enzyme is a type of protein that acts as a
catalyst, speeding up chemical reactions
Substrate
(sucrose)
Enzyme
(sucrose)
Fructose
Glucose
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● consist of 1 or more polypeptide
chains folded and coiled into
specific conformations
● Polypeptide chains =
polymers of amino acids
– arranged in specific linear sequence
– linked by PEPTIDE BONDS
– range in length from a few to 1000+
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AMINO ACIDS
● building blocks of proteins
● structure of an amino acid
– central carbon
– hydrogen atom
– carboxyl group
– amino group
– variable R group (side chain)
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Amino Acid Monomers
● Cells use 20 amino acids to make
thousands of proteins
Amino
group
Carboxyl
group
a carbon
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● linked together by PEPTIDE BONDS
(links the carboxyl group of 1 amino
acid to the amino group of another;
requires dehydration / condensation)
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PROTEIN STRUCTURE
● a protein’s function depends on its
specific conformation (3-D shape)
● Four levels of protein structure:
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1. Primary structure
● unique, linear sequence of
amino acids (forms the
“backbone” of a protein)
– determined by genes
– a slight change can affect a
protein’s conformation and
function (e.g. sickle-cell
hemoglobin)
– can be sequenced in the
laboratory (insulin)
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2. Secondary structure
● regular, repeated coiling and folding
of a protein’s polypeptide backbone
● stabilized by H-bonds between
peptide linkages in protein’s
backbone (NOT the amino acid side
chains)
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● ALPHA HELIX = helical coil stabilized
by H-bonding between every 4th peptide
bond (3.6 amino acid/turn)
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● BETA PLEATED SHEET = sheet of
antiparallel chains folded into
“accordion” pleats
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Spiders secrete silk fibers made of a structural
protein containing beta-pleated sheets…able to
stretch & recoil!
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3. Tertiary Structure
● irregular
contortions of
protein due to
bonding between
side chains (R
groups) resulting
in 3-D shape
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Hydrophobic
interactions and
van der Waals
interactions
Polypeptide
backbone
Disulfide bridge
Ionic bond
Hydrogen
bond
Tertiary
Protein
Structure
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4. Quaternary Structure
● association of 2 or more protein
subunits to form a single
functioning molecule (i.e.
hemoglobin and collagen)
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b Chains
a ChainsHemoglobin
Iron
Heme
CollagenPolypeptide chain
Polypeptidechain
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Protein Form and Function
● A functional protein consists of one or more
polypeptides twisted, folded, and coiled into a
unique shape
● The sequence of amino acids determines a
protein’s three-dimensional formation
● A protein’s formation
determines its function
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Protein Conformation
● determined by physical and chemical environmental conditions
● DENATURATION: process that alters a protein’s native conformation and hence its biological activity
– heat
– salt concentration
– chemical agents that disrupt H-bonding
– transfer to an organic solvent
– pH changes
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(SDS is a
detergent that
denatures
proteins.)
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Protein Denaturation:
● A denatured protein is misshapen
and therefore biologically inactive
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The Protein-Folding Problem
● It is hard to predict a protein’s conformation
from its primary structure
● Most proteins probably go through several
states on their way to a stable conformation
● Chaperonins are protein molecules that assist
the proper folding of other proteins
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Chaperonin
(fully assembled)
Hollow
cylinder
Cap
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Polypeptide
Correctly
folded
protein
An unfolded poly-
peptide enters the
cylinder from one
end.
Steps of Chaperonin
Action:The cap comes
off, and the
properly folded
protein is released.
The cap attaches, causing
the cylinder to change
shape in such a way that
it creates a hydrophilic
environment for the
folding of the polypeptide.
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5.5 - Nucleic acids store and
transmit hereditary information
● The amino acid sequence of a
polypeptide is programmed by a
unit of inheritance called a GENE
● Genes are made of DNA, a nucleic
acid
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● Two types of nucleic acids:
1) DNA
2) RNA
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The Roles of Nucleic Acids
● There are two types of nucleic acids:
-Deoxyribonucleic acid (DNA)
-Ribonucleic acid (RNA)
● DNA directs synthesis of messenger RNA
(mRNA) and, through mRNA, controls protein
synthesis
● Protein synthesis occurs in ribosomes
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1. DNA = Deoxyribonucleic acid
● encodes the instructions for amino acid
sequences of proteins
● is copied and passed from one generation
of cells to another
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2. RNA = Ribonucleic acid
● functions in the actual
synthesis of proteins
coded for by DNA
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● carries the encoded information to the
ribosomes; carries the amino acids to the
ribosome; a major component of ribosomes
DNA RNA Protein
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Structure of Nucleic Acids
● polymers of monomers called
NUCLEOTIDES
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● Each nucleotide consists of:
1. Pentose (5-carbon sugar)
-deoxyribose in DNA
-ribose in RNA
2. Phosphate group (attached to #5 carbon
on sugar)
3. Nitrogenous base-purines (double ring)-pyrimidines (single ring)
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The Structure of Nucleic Acids
● The portion of a nucleotide
without the phosphate
group is called a
nucleoside
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● nucleotides are joined
together by phosphodiester
linkages (between
phosphate of one nucleotide
and the sugar of the next)
● results in a backbone with a
repeating pattern of sugar-
phosphate-sugar-
phosphate...
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5 end
3 end
Nucleoside
Nitrogenous
base
Phosphate
group
Nucleotide
Polynucleotide, or
nucleic acid
Pentose
sugar
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The DNA Double Helix
● A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix
● One DNA molecule includes many genes
● The nitrogenous bases in DNA form hydrogen bonds in a complementary fashion:
A always with T, and
G always with C
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DNA & Proteins as Tape
Measures of Evolution● genes and their products (proteins)
document the hereditary background of an organism
● linear sequences of DNA are passed from parents to offspring; 2 siblings have greater similarity in their DNA than do unrelated individuals…
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DNA & Proteins as Tape
Measures of Evolution● it follows, that 2 closely related species
would share a greater proportion of their DNA & protein sequences than 2 distantly related species would…
● that is the case!!!
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DNA & Proteins as Tape
Measures of Evolution● example: the β chain of human
hemoglobin:
● this chain contains 146 amino acids
-humans & gorillas differ in 1 amino acid
-humans & frogs differ in 67 amino acids
● Molecular biology has added a new “tape measure” with which we can study evolutionary relationships!!
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