Chem 45 Biochemistry: Stoker Chapter 20 Proteins

Chapter 20
Table of Contents
Copyright © Cengage Learning. All rights reserved 2
20.1 Characteristics of Proteins
20.2 Amino Acids: The Building Blocks for Proteins
20.3 Essential Amino Acids
20.4 Chirality and Amino Acids
20.5 Acid–Base Properties of Amino Acids
20.6 Cysteine: A Chemically Unique Amino Acid
20.7 Peptides
20.8 Biochemically Important Small Peptides
20.9 General Structural Characteristics of Proteins
20.10 Primary Structure of Proteins
20.11 Secondary Structure of Proteins
20.12 Tertiary Structure of Proteins
20.13 Quaternary Structure of Proteins
20.14 Protein Hydrolysis
20.15 Protein Denaturation
20.16 Protein Classification Based on Shape
20.17 Protein Classification Based on Function
20.18 Glycoproteins
20.19 Lipoproteins

Characteristics of Proteins
Section 20.1
• A protein is a naturally-occurring, unbranched polymer in
which the monomer units are amino acids
• Proteins are most abundant molecules in the cells after
water – account for about 15% of a cell’s overall mass
• Elemental composition - Contain Carbon (C), Hydrogen
(H), Nitrogen (N), Oxygen (O), and Sulfur (S)
• The average nitrogen content of proteins is 15.4% by
mass
• Also present are Iron (Fe), phosphorus (P) and some
other metals in some specialized proteins

Section 20.1
• General definition: A protein is a naturally-occurring, unbranched
polymer in which the monomer units are amino acids.
• Specific definition: A protein is a peptide in which at least 40 amino
acid residues are present:
– The terms polypeptide and protein are often used
interchangeably to describe a protein
– Several proteins with >10,000 amino acid residues are known
– Common proteins contain 400–500 amino acid residues
– Small proteins contain 40–100 amino acid residues
• More than one polypeptide chain may be present in a protein:
– Monomeric : Contains one polypeptide chain
– Multimeric: Contains 2 or polypeptide chains

Section 20.1
Protein Classification Based on Chemical Composition
• Simple proteins:
• A protein in which only amino acid residues are present:
– More than one protein subunit may be present but all subunits
contain only amino acids
Albuminoids - keratin in skin, hair, nails; collagen in
cartilage
Albumins - egg albumin, serum albumin
Globulins - antibodies
Histones - chromatin in chromosomes

Section 20.1
Conjugated (complex) proteins:
A protein that has one or more non-amino acid entities (prosthetic groups) present in its structure:
Prosthetic group may be organic or inorganic
Protein Classification Based on Chemical Composition

Section 20.1
Fibrous Proteins: Alpha-Keratin & Collagen
Protein Classification Based on Shape
Globular Proteins: Myoglobin & Hemoglobin
• The polypeptide chains are arranged in long strands or sheets
• Have long, rod-shaped or string-like molecules that can intertwine
with one another and form strong fibers; water-insoluble
• Structural functions
• The polypeptide chains are folded into spherical or globular shapes
• Nonpolar amino acids are in the interior, polar amino acids are on the
surface
• Water-soluble which allows them to travel through the blood and other
body fluids to sites where their activity is needed
• Dynamic functions

Section 20.1
Table 20-6 p735

Section 20.1
• Proteins play crucial roles in most biochemical
processes.
• The diversity of functions exhibited by proteins far
exceeds the role of other biochemical molecules
• The functional versatility of proteins stems from:
– Ability to bind small molecules specifically and
strongly
– Ability to bind other proteins and form fiber-like
structures, and
– Ability integrated into cell membranes
Protein Classification Based on Function

Section 20.1
Major Categories of Proteins Based on Function
• Catalytic proteins: Enzymes are best known for their catalytic role.
– Almost every chemical reaction in the body is driven by an
enzyme
• Defense proteins: Immunoglobulins or antibodies are central to
functioning of the body’s immune system.
• Transport proteins: Bind small biomolecules, e.g., oxygen and other
ligands, and transport them to other locations in the body and
release them on demand.
• Messenger proteins: transmit signals to coordinate biochemical
processes between different cells, tissues, and organs.
– Insulin and glucagon - regulate carbohydrate metabolism
– Human growth hormone – regulate body growth

Section 20.1
• Contractile proteins: Necessary for all forms of movement.
– Muscles contain filament-like contractile proteins (actin and
myosin).
– Human reproduction depends on the movement of sperm –
possible because of contractile proteins.
• Structural proteins: Confer stiffness and rigidity
– Collagen is a component of cartilage
– Keratin gives mechanical strength as well as protective covering
to hair, fingernails, feathers, hooves, etc.
• Transmembrane proteins: Span a cell membrane and help control
the movement of small molecules and ions.
– Have channels – help molecules to enter and exist the cell.
– Transport is very selective - allow passage of one type of
molecule or ion.

Section 20.1
• Storage proteins: Bind (and store) small molecules.
– Ferritin - an iron-storage protein - saves iron for use in the
biosynthesis of new hemoglobin molecules.
– Myoglobin - an oxygen-storage protein present in muscle
• Regulatory proteins: Often found “embedded” in the exterior surface
of cell membranes - act as sites for receptor molecules
– Often the molecules that bind to enzymes (catalytic proteins),
thereby turning them “on” and “off,” and thus controlling
enzymatic action.
• Nutrient proteins: Particularly important in the early stages of life -
from embryo to infant.
– Casein (milk) and ovalalbumin (egg white) are nutrient proteins
– Milk also provide immunological protection for mammalian
young.

Section 20.2
Amino Acids: The Building Blocks for Proteins
• Amino acid - An organic compound that contains both an amino (-NH2) and
a carboxyl (-COOH) group attached to same carbon atom
– The position of carbon atom is Alpha (a)
– -NH2 group is attached at alpha (a) carbon atom.
– -COOH group is attached at alpha (a) carbon atom.
– R = side chain –vary in size, shape, charge, acidity, functional groups
present, hydrogen-bonding ability, and chemical reactivity.
– >700 amino acids are known
– Based on common “R” groups, there are 20 standard amino acids

Section 20.2
Nomenclature
• Common names assigned to the amino acids are currently used.
• Three letter abbreviations - widely used for naming:
– First letter of amino acid name is compulsory and capitalized
followed by next two letters not capitalized except in the case of
Asparagine (Asn), Glutamine (Gln) and tryptophan (Trp).
• One-letter symbols - commonly used for comparing amino acid
sequences of proteins:
– Usually the first letter of the name
– When more than one amino acid has the same letter the most
abundant amino acid gets the 1st letter.

Section 20.2
• All amino acids differ from one another by their R-groups
– There are 20 common (standard) amino acids
• Standard amino acids are divided into four groups based
on the properties of R-groups
• Non-polar amino acids: R-groups are non-polar
– Such amino acids are hydrophobic-water fearing
(insoluble in water)
– 8 of the 20 standard amino acids are non polar
– When present in proteins, they are located in the
interior of protein where there is no polarity

Section 20.2
Non-Polar Amino Acids

Section 20.2
• Polar amino acids: R-groups are polar
– Three types: Polar neutral; Polar acidic; and Polar
basic
• Polar-neutral: contains polar but neutral side chains
– Seven amino acids belong to this category
• Polar acidic: Contain carboxyl group as part of the side
chains
– Two amino acids belong to this category
• Polar basic: Contain amino group as part of the side
chain
– Two amino acids belong to this category

Section 20.2
Polar Neutral Amino Acids

Section 20.2
Polar Acidic and Basic Amino Acids

Section 20.2
Practice Exercise
• Classify the following amino acids based on the polarity of their R-groups

Section 20.2
Practice Exercise
• Classify the following amino acids based on the polarity of their R-groups
Non-polar
Non-polar
Polar Neutral
Polar Acidic
Polar Basic

Section 20.2
• Derived amino acids (“nonstandard” amino acids)
• usually formed by an enzyme-facilitated reaction on a common amino acid
after that amino acid has been incorporated into a protein structure
• examples are cystine, desmosine, and isodesmosine found in elastin
hydroxyproline, and hydroxylysine found in collagen
-carboxyglutamate found in prothrombin

Section 20.3
Essential Amino Acids
• Essential Amino acid: A standard amino acid needed for
protein synthesis that must be obtained from dietary
sources – adequate amounts cannot be synthesized in
human body.
• Nine of the 20 standard amino acids are considered
essential
Arginine* Methionine
Histidine Phenylalanine
Isoleucine Threonine
Leucine Tryptophan
Lysine Valine *Required for growth in children
and is not essential for adults.

Section 20.3
Some of the first informations on the biological value of dietary proteins came from
studies on rats. In one series of experiments, young rats were fed diets containing 18%
protein in the form of either casein (a milk protein), gliadin (a wheat protein), or zein (a
corn protein)
Results:
Casein: rats remained healthy and grow normally
Gliadin: rats maintained their weight but did not grow much
Zein: rats not only failed to grow but began to lose weight, &
eventually died if kept on this diet
• Since casein evidently supplies all the required amino acids in the correct
proportions needed for growth, it is called a complete protein.
• A complete protein contains all the essential amino acids in the proper amounts.
• An incomplete protein is low in one or more of the essential amino acids, usually
lysine, tryptophan, or methionine. Except for gelatin, proteins from animal sources
are complete, whereas proteins from vegetable sources are incomplete except soy
protein.
• Complementary proteins are incomplete proteins which when served together,
complement each other and provide all the essential amino acids

Section 20.4
Chirality and Amino Acids
• Four different groups are attached to
the a-carbon atom in all of the
standard amino acids except glycine
– In glycine R-group is hydrogen
• Therefore 19 of the 20 standard
amino acids contain a chiral center
• Molecules with chiral centers exhibit
enantiomerism (left- and right-
handed forms)
• The amino acids found in nature as
well as in proteins are L isomers.
– Bacteria do have some D-amino
acids
– With monosaccharides nature
favors D-isomers

Section 20.4
Practice Exercise
H2N C
COOH
CH2
OH
HNH2C
COOH
H
CH2
SH
CH CH3
CH2
CH3
H2N C
COOH
H
** *
A B C
• Name the following amino acids with correct designation for
the enantiomer (chiral carbon is indicated by *).

Section 20.4
Practice Exercise
H2N C
COOH
CH2
OH
HNH2C
COOH
H
CH2
SH
CH CH3
CH2
CH3
H2N C
COOH
H
** *
A = L-Isoleucine
B = D-Cysteine
C = L-Tyrosine
A B C
• Name the following amino acids with correct designation for
the enantiomer (chiral carbon is indicated by *).

Section 20.5
Acid–Base Properties of Amino Acids
• In pure form amino acids are white crystalline solids
• Most amino acids decompose before they melt
• Not very soluble in water
• Under physiological conditions exists as Zwitterions: An ion with +
(positive) and – (negative) charges on the same molecule with a net
zero charge
– Carboxyl groups give-up a proton to get negative charge
– Amino groups accept a proton to become positive

Section 20.5
• Amino acids in solution exist in three different species (zwitterions,
positive ion, and negative ion) - Equilibrium shifts with change in pH
• Isoelectric point (pI) – pH at which the concentration of Zwitterion is
maximum -- net charge is zero
– Different amino acids have different isoelectric points
– At isoelectric point - amino acids are NOT attracted towards an
applied electric field because they carry net zero charge.
Isoelectric Point (pI)

Section 20.5
• At pH values lower than pI, the carboxylate end of the zwitterion
picks up a proton from solution, the amino acid acquires a net +
charge, and migrate to the negative electrode in an electric field.
• At pH values higher than pI, a proton is removed from the
ammonium end of the zwitterion, the amino acid acquires a net
negative(-) charge and migrate to the negative electrode

Section 20.5
Drill: For each amino acid: ala (pK
1
= 2.19; pK
2
= 9.67),
asp (pK
1
= 2.09; pK
2
= 9.82;
pK
R
= 3.86)
lys (pK
1
= 2.18; pK
2
= 8.95;
pK
R
= 10.79)
a). Draw the structure of the amino acid as the pH of the
solution changes from highly acidic to strongly
basic.
b) Which form of the amino acid is present at isoelectric
point?
c) Calculate the isoelectric point, pI ; i.e., the average of the
two pKa’s bracketing the isoelectric structure.

Section 20.7
Peptides
• Under proper conditions, amino acids can bond together to produce
an unbranched chain of amino acids.
– The reactions is between amino group of one amino acid and
carboxyl group of another amino acid.
• The length of the amino acid chain can vary from a few amino acids
to hundreds of amino acids.
• Such a chain of covalently-linked amino acids is called a peptide.
• The covalent bonds between amino acids in a peptide are called
peptide bonds (amide).
Nature of Peptide Bond

Section 20.7
Peptides
• by convention, the structure of peptides is represented beginning
with the amino acid whose amino group is free (N-terminal end).
The other end contains a free carboxyl group and is the C-terminal
end
• amino acids are added to a peptide by forming peptide bonds with
the C-terminal amino acid
+
H3N CH C
CH3
O
H
N CH C
CH2
O
H
N CH C
CH2
O-
O
OH
Alanine Phenylalanine Serine
C-terminal end
N-terminal end

Section 20.7
Peptides
• the C – N bond in the peptide
linkage has partial double bond
character that makes it rigid and
prevents the adjacent groups
from rotating freely; the peptide
bond is planar, and the two
adjacent a-carbons lie trans to it.
The H of the amide nitrogen is
also trans to the O of the
carbonyl group. Almost all of the
peptide bonds in proteins are
planar and have a trans
configuration.

Section 20.7
Peptides
Peptide Nomenclature
• The C-terminal amino acid
residue keeps its full amino acid
name.
• All of the other amino acid
residues have names that end
in -yl. The -yl suffix replaces the
-ine or -ic acid ending of the
amino acid name, except for
tryptophan, for which -yl is
added to the name.
• The amino acid naming
sequence begins at the N-
terminal amino acid residue.
• Example:
– Ala-leu-gly has the IUPAC
name of alanylleucylglycine

Section 20.7
Peptides
Isomeric Peptides
• Peptides that contain the same amino acids but present
in different order are different molecules (constitutional
isomers) with different properties
– For example, two different dipeptides can be formed
between alanine and glycine
• The number of isomeric peptides possible increases
rapidly as the length of the peptide chain increases

Section 20.7
Peptides
Drill
• Write the structure of the tetrapeptide lys-ser-asp-ala at
the isoelectric point. Calculate its pI.
• pKa values: Lysine Ser Asp Ala
pK1 2.18 2.21 2.10 2.34
pK2 8.95 9.15 9.82 9.87
pKR 10.79 - 3.86 -

Section 20.8
Biochemically Important Small Peptides
• Many relatively small peptides are biochemically active:
– Hormones - Artificial sweeteners
– Neurotransmitters
– Antioxidants
• Small Peptide Hormones:
– Best-known peptide hormones: oxytocin and vasopressin
– Produced by the pituitary gland
– Nonapeptide (nine amino acid residues) with six of the residues held in
the form of a loop by a disulfide bond formed between two cysteine
residues

Section 20.8
Small Peptide Neurotransmitters
• Enkephalins are pentapeptide neurotransmitters
produced by the brain and bind receptors within the brain
• Help reduce pain
• Best-known enkephalins:
– Met-enkephalin: Tyr–Gly–Gly–Phe–Met
– Leu-enkephalin: Tyr–Gly–Gly–Phe–Leu

Section 20.8
Small Peptide Antioxidants
• Glutathione (Glu–Cys–Gly) – a tripeptide – is present is in high levels in
most cells
• Regulator of oxidation–reduction reactions.
• Glutathione is an antioxidant and protects cellular contents from oxidizing
agents such as peroxides and superoxides
– Highly reactive forms of oxygen often generated within the cell in
response to bacterial invasion
• Unusual structural feature – Glu is bonded to Cys through the side-chain
carboxyl group.

Section 20.8
Small Peptide Artificial Sweeteners
• Aspartame (Asp-Phe) - dipeptide sold under trade
names Equal and Nutrasweet; ~180x as sweet as
sucrose

Section 20.10
Primary Structure of Proteins
Four Types of Structures
– Primary Structure
– Secondary Structure
– Tertiary Structure
– Quaternary
• Primary Structure is the order in
which amino acids are linked
together in a protein by peptide
bonds;
• the backbone of the protein
molecule
• Every protein has its own unique
amino acid sequence
– Frederick Sanger (1953) sequenced
and determined the primary
structure for the first protein - Insulin

Section 20.10
The order in which the amino acid residues of a peptide molecule are linked is the amino acid
sequence of the molecule; differences in the chemical and physiologic properties of peptides result
from differences in the amino acid sequence.
e.g., a) Bradykinin vs Boguskinin
- partly responsible for triggering pain, - completely inactive, hence, the name
welt formation (as in scratches), bogus or false
movement of smooth muscle, and
lowering of blood pressure
Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe
Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg
b) Normal Hb: …..Val-His-Leu-Thr-Pro-Glu-Glu-Lys-Ser-Ala-…..
Sickle-cell Hb: …..Val-His-Leu-Thr-Pro-Val-Glu-Lys-Ser-Ala-…..
Georgetown anemia: …..Val-His-Leu-Thr-Pro-Glu-Lys-Lys-Ser-Ala-…..

Section 20.10
Sequencing a Protein
• Steps:
1. Hydrolysis – effected by acid, alkali, or enzyme
2. Identification of the products of hydrolysis
3. Fitting the pieces together as in a jigsaw puzzle
A. Acid Hydrolysis – heating in presence of 6M HCl at 110oC,10-100 h
destroys Trp
B. Alkaline Hydrolysis – heating in presence of 4M NaOH; does not
damage Trp but destroys a lot more amino acids
C. Enzymatic Hydrolysis – use proteases/peptidases

Section 20.10
Sequencing a Protein
Enzymatic Hydrolysis
1) Exopeptidases – cleaves external peptide bonds
a) aminopeptidase – sequentially cleave peptide bonds beginning
at the N-terminal end
b) carboxypeptidase – sequentially cleave peptide bonds beginning
at the C-terminal end
2) Endopeptidases – cleaves internal peptide bonds
a) trypsin – cleaves peptide bonds at the carboxyl end of two
strongly basic aa’s : Arg & Lys
b) chymotrypsin – cleaves peptide bonds at the carboxyl end of the
three aromatic aa’s: Phe, Tyr, & Trp; Leu
c) pepsin – cleaves peptide bonds at the amino end of the three
aromatic aa’s: Phe, Tyr, & Trp; Ileu

Section 20.10
Chemical Methods of Determining the N-terminal end and the
C-terminal end
• A. N-terminal end
a) Edman’s method
phenylisothiocyanate,
PITC, (Ph – N = C = S)
combines with the N-
terminal amino acid to
yield a phenylthiohydantoin-
compound (PTH-amino
acid), which may be
identified by
chromatography. PTh-
amino acid can be extracted
by organic solvent

Section 20.10
Chemical Methods of Determining the N-terminal end and the
C-terminal end
• A. N-terminal end
b) Sanger’s method
2,4-dinitrofluorobenzene (DNFB)
binds to the N-terminal amino group of
the polypeptide. The resultant
dinitrophenyl-amino acid or DNP-aa
can be separated from the other
amino acids because it is more
soluble in nonpolar solvents
• B. C-terminal end
Hydrazine method
hydrazine reacts with all amino
acids whose carboxyl group is
bound in peptide linkage, creating
amino acyl hydrazides. Only the C-
terminal amino acid is spared

Section 20.10
Drills
1. Write the peptides generated from chymotrypsin, trypsin, and pepsin
digestion of
Ala-His-Tyr-Pro-Trp-Arg-Ileu-Phe-Glu-Lys-Cys
2. Total acid hydrolysis of a pentapeptide complemented by total alkaline
hydrolysis yields an equimolqr mixture of five amino acids: ala, cys, lys,
phe, and ser. N-terminal analysis with PITC generates PTH-serine.
Trypsin digestion produces a tripeptide whose N-terminal residue is cys
and a dipeptide with ser at its N-terminal. Chymotrypsin digestion of the
above tripeptide yields ala plus another dipeptide. (a) What is the amino
acid sequence of the tripeptide? (b) What is the amino acid composition of
the dipeptide derived from trypsin digestion? (c) What is the primary
structure of the original pentapeptide?

Section 20.11
Secondary Structure of Proteins
• Arrangement of atoms of peptide backbone in space.
• The peptide linkages are essentially planar thus allows only two possible
arrangements for the peptide backbone for the following reasons:
– For two amino acids linked through a peptide bond six atoms lie in the same
plane
– The planar peptide linkage structure has considerable rigidity, therefore rotation
of groups about the C–N bond is hindered
– Cis–trans isomerism is possible about C–N bond.
– The trans isomer is the preferred orientation
The only bond responsible for the 2o structure of proteins is H-bonding
between peptide bonds, the – C = O of one peptide group and the – N – H
of another peptide linkage farther along the backbone.
The two most common types : alpha-helix (a-helix) and the beta-pleated sheet
(b-pleated sheet).

Section 20.11
Alpha-helix (α-helix)
• A single protein chain adopts a
shape that resembles a coiled
spring (helix):
– H-bonding between amino
acids with in the same
chain –intramolecular H-
bonding
– Coiled helical spring
– R-groups stay outside of
the helix -- not enough
room for them to stay
inside
– The helix is so tightly
wound that the space in
the center is too small for
solvent molecules to enter

Section 20.11
Beta-Pleated Sheets
• Completely extended
protein chain segments in
same or different molecules
governed by intermolecular
or intramolecular H-bonding
• H-bonding between different
parts of a single chain –
intramolecular H-bonding
• Side chains below or above
the axis
• “U-turn” structure – most
frequently encountered
Copyright © Cengage Learning. All rights reserved
51

Section 20.11
Beta-Pleated Sheets
• Completely extended
protein chain segments in
same or different molecules
• H-bonding between different
chains – intermolecular H-
bonding
a) parallel – chains run in
the same direction
b) antiparallel – chains run
in opposite direction;
more stable because
of fully collinear H-
bonds

Section 20.12
Tertiary Structure of Proteins
• The overall three-dimensional shape of a protein
• Results from the interactions between amino acid side chains (R
groups) that are widely separated from each other.
• Defines the biological function of proteins
• Proteins may have, in general, either of the two forms of tertiary
structures: (a) fibrous and (b) globular
fibrous proteins (insoluble): mechanical strength, structural
components, movement
globular proteins (soluble): transport, regulatory, enzymes
• In general 4 types of interactions are observed.
– Disulfide bonding
– Electrostatic interactions
– H-Bonding
– Hydrophobic interactions

Section 20.12
Four Types of Interactions
• Disulfide bond: covalent,
strong, between two cysteine
groups; the strongest of the 3o
bonds
• Link chains together and
cause chains to twist and
bend

Section 20.12
• Electrostatic interactions (ionic
interaction, salt linkages): Salt
Bridge between charged side
chains of acidic and basic
amino acids
– -OH, -NH2, -COOH, -
CONH2
• H-Bonding between polar,
acidic and/or basic R groups
– For H-bonding to occur,
the H must be attached to
O, N or F

Section 20.12
• Hydrophobic interactions:
Between non-polar side
chains

Section 20.12
• Drill

Section 20.13
Quaternary Structure of Proteins
• Quaternary structure of protein refers to the organization among the
various polypeptide chains in a multimeric protein:
• Highest level of protein organization
• Present only in proteins that have 2 or more polypeptide chains
(subunits)
• Subunits are generally independent of each other - not
covalently bonded
• Proteins with quaternary structure are often referred to as
oligomeric proteins
• Contain even number of subunits

Section 20.13

Section 20.13
Fibrous Proteins: Alpha-Keratin
• Provide protective coating for organs
• Major protein constituent of hair, feather, nails, horns and turtle
shells
• Mainly made of hydrophobic amino acid residues
• Hardness of keratin depends upon -S-S- bonds
– More –S-S– bonds make nail and bones hard and hair brittle

Section 20.13
Fibrous Proteins: Collagen
• Most abundant proteins in humans (30% of total body protein)
• Major structural material in tendons, ligaments, blood vessels, and
skin
• Organic component of bones and teeth
• Predominant structure - triple helix
• Rich in proline (up to 20%) – important to maintain structure

Section 20.13
Globular Proteins: Myoglobin
• Globular Proteins: Myoglobin:
– An oxygen storage molecule in muscles.
– Monomer - single peptide chain with one heme unit
– Binds one O2 molecule
– Has a higher affinity for oxygen than hemoglobin.
– Oxygen stored in myoglobin molecules serves as a reserve
oxygen source for working muscles

Section 20.13
Globular Proteins: Hemoglobin
• An oxygen carrier molecule in blood
• Transports oxygen from lungs to tissues
• Tetramer (four polypeptide chains) - each subunit has a heme group
• Can transport up to 4 oxygen molecules at time
• Iron atom in heme interacts with oxygen

Section 20.18
Glycoproteins
• Conjugated proteins with carbohydrates linked to them:
– Many of plasma membrane proteins are glycoproteins
– Blood group markers of the ABO system are also glycoproteins
– Collagen and immunoglobulins are glycoproteins
• Collagen -- glycoprotein
– Most abundant protein in human body (30% of total body
protein)
– Triple helix structure
– Rich in 4-hydroxyproline (5%) and 5-hydroxylysine (1%) —
derivatives
– Some hydroxylysines are linked to glucose, galactose, and their
disaccharides – help in aggregation of collagen fibrils.

Section 20.18
Glycoproteins
Immunoglobulins
• Glycoproteins produced as a protective response to the invasion of
microorganisms or foreign molecules - antibodies against antigens.
• Immunoglobulin bonding to an antigen via variable region of an
immunoglobulin occurs through hydrophobic interactions, dipole –
dipole interactions, and hydrogen bonds.
• Carbohydrate molecules attached to the heavy chains aid in
determining the destinations of immunoglobulins in the tissues

Section 20.19
Lipoproteins
• Lipoprotein: a conjugated protein that contains lipids in addition to
amino acids
• Major function - help suspend lipids and transport them through the
bloodstream
• Four major classes of plasma lipoproteins:
– Chylomicrons: Transport dietary triacylglycerols from intestine
to liver and to adipose tissue.
– Very-low-density lipoproteins (VLDL): Transport triacylglycerols
synthesized in the liver to adipose tissue.
– Low-density lipoproteins (LDL): Transport cholesterol
synthesized in the liver to cells throughout the body.
– High-density lipoproteins (HDL): Collect excess cholesterol from
body tissues and transport it back to the liver for degradation to
bile acids.

Section 20.15
Protein Denaturation
• Partial or complete disorganization of
protein’s tertiary structure
• Cooking food denatures the protein but
does not change protein nutritional value
• Coagulation: Precipitation (denaturation
of proteins)
– Egg white - a concentrated solution
of protein albumin - forms a jelly
when heated because the albumin
is denatured
• Cooking:
– Denatures proteins – Makes it easy
for enzymes in our body to
hydrolyze/digest protein
– Kills microorganisms by
denaturation of proteins
– Fever: >104ºF – the critical
enzymes of the body start getting
denatured

Section 20.15

Chem 45 Biochemistry: Stoker Chapter 20 Proteins

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Chem 45 Biochemistry: Stoker Chapter 20 Proteins