ProteinProtein

ProteinsProteins: Complex molecules made of : Complex molecules made of hydrogen, carbon, oxygen, nitrogen and hydrogen, carbon, oxygen, nitrogen and sometimes other elements.sometimes other elements.– Because of their size, proteins are often called Because of their size, proteins are often called

macromoleculesmacromolecules MacromoleculeMacromolecule: A large molecule containing many : A large molecule containing many

atomsatoms

Nitrogen plays an important part in Nitrogen plays an important part in proteins.proteins.– It sets proteins apart from carbohydrates or It sets proteins apart from carbohydrates or

lipids, which do not contain nitrogen.lipids, which do not contain nitrogen.

Structure of ProteinsStructure of Proteins Protein is made of chains of substances called Protein is made of chains of substances called

amino acidsamino acids: a type of organic acid. : a type of organic acid. – Organic acidsOrganic acids are molecules that contain a carboxyl are molecules that contain a carboxyl

group (-COOH). group (-COOH). – They also contain an They also contain an amine groupamine group: two atoms of : two atoms of

hydrogen and one atom of nitrogen (-NH2).hydrogen and one atom of nitrogen (-NH2).

The carboxyl group and the amine The carboxyl group and the amine group are attached to a central group are attached to a central carbon in the carbon in the

amino acid. amino acid. A third bond to this carbon is to a A third bond to this carbon is to a

single hydrogen. single hydrogen. One bond is left open for a variable One bond is left open for a variable

side chain, making each amino acid side chain, making each amino acid different than the others.different than the others.

20 Amino Acids20 Amino Acids

Orange: Hydrophobic

Green: Hydrophilic

Pink: Acidic

Turquoise: Basic

Peptide BondsPeptide Bonds

Peptide bondsPeptide bonds: The bonds : The bonds between the nitrogen of one amino between the nitrogen of one amino acid and the carbon of a second acid and the carbon of a second amino acid.amino acid.– The peptide bonds link to form a protein The peptide bonds link to form a protein

molecule.molecule. When one hydrogen in one amino acid joins When one hydrogen in one amino acid joins

with the –OH of another amino acid, water is with the –OH of another amino acid, water is formed, and released, leaving a peptide formed, and released, leaving a peptide bond.bond.

PolypeptidesPolypeptides

Through peptide bonds, amino acids Through peptide bonds, amino acids chain together, creating a chain together, creating a polypeptidepolypeptide: : a single protein molecule containing ten a single protein molecule containing ten or more amino acids linked in peptide or more amino acids linked in peptide chains.chains.– Some protein chains contain only a few Some protein chains contain only a few

amino acids. Most molecules contain 100 to amino acids. Most molecules contain 100 to 500 amino acids, and some have thousands.500 amino acids, and some have thousands. The polypeptide chain is not straight; it coils, folds The polypeptide chain is not straight; it coils, folds

and tangles.and tangles.

Protein StructuresProtein Structures

Structure and Function of Structure and Function of ProteinsProteins

A protein’s shape determines its function.A protein’s shape determines its function.– Protein molecules that form rope-like fibers are Protein molecules that form rope-like fibers are

called called fibrous proteinfibrous protein.. This structure strengthens the fiber to serve as This structure strengthens the fiber to serve as

connective tissue in the body.connective tissue in the body.– Ex: Collagen and elastin are fibrous proteins.Ex: Collagen and elastin are fibrous proteins.

– Protein molecules that form a structure that Protein molecules that form a structure that can be compared to steel wool are called can be compared to steel wool are called globular proteinsglobular proteins. . Their rounded shape makes them convenient carriers.Their rounded shape makes them convenient carriers.

– Ex: The protein, hemoglobin transports oxygen to the Ex: The protein, hemoglobin transports oxygen to the blood. blood.

Fibrous Protein: Connective tissue, tendons, bone matrix, muscle fiber

NOT SOLUBLE IN WATER

Globular Protein: Hemoglobin, enzymatic catalysis, hormones (messengers), transporters through membranes

SOLUBLE IN WATER

Denaturation of ProteinDenaturation of Protein

Denaturation: The process of Denaturation: The process of changing the shape of a protein changing the shape of a protein molecule without breaking its molecule without breaking its peptide bonds.peptide bonds.– Denaturation breaks the hydrogen Denaturation breaks the hydrogen

bonds that create the twists and turns of bonds that create the twists and turns of a protein molecule.a protein molecule. The result is a looser, less compact The result is a looser, less compact

structure, changing the original properties of structure, changing the original properties of the protein.the protein.

CoagulationCoagulation

Denaturation is the first step in the Denaturation is the first step in the process of process of coagulationcoagulation: Changing a : Changing a liquid into a soft, semisolid clot or liquid into a soft, semisolid clot or solid mass.solid mass.– It occurs when polypeptides unfold It occurs when polypeptides unfold

during denaturation, then collide and during denaturation, then collide and clump together to form a solid.clump together to form a solid. Ex: Scrambled eggs; beating the eggs Ex: Scrambled eggs; beating the eggs

denatures the protein, then the protein denatures the protein, then the protein coagulates as the egg cooks.coagulates as the egg cooks.

Denaturation by HeatDenaturation by Heat

Heat is the most common agent used Heat is the most common agent used to denature protein.to denature protein.– Temperature is a significant factor.Temperature is a significant factor.

The rate and the degree of denaturation The rate and the degree of denaturation increase 600 times for every 10 degrees C increase 600 times for every 10 degrees C rise in temperature.rise in temperature.

– The structure of the protein affects the The structure of the protein affects the process also.process also. Most proteins denature at temps between 47 Most proteins denature at temps between 47

and 67 degrees C, as in eggs and milk. and 67 degrees C, as in eggs and milk. – The denaturation of beef takes a much higher The denaturation of beef takes a much higher

temp.temp.

Other Means of Other Means of DenaturationDenaturation

Proteins may denature due to the reaction of:Proteins may denature due to the reaction of:– Freezing, pressure, sound waves, and the addition Freezing, pressure, sound waves, and the addition

of certain compounds.of certain compounds.– Mechanical treatment, as in beating eggs and Mechanical treatment, as in beating eggs and

kneading bread.kneading bread.– Very high and very low pH. Adding lemon juice Very high and very low pH. Adding lemon juice

can sour milk; milk proteins denature, coagulate can sour milk; milk proteins denature, coagulate and separate from the liquid.and separate from the liquid.

– Certain metal ions. Sodium and potassium ions Certain metal ions. Sodium and potassium ions are most commonly used, but copper and iron will are most commonly used, but copper and iron will have the same effect.have the same effect.

Denaturation of proteins involves the disruption and possible destruction of both the secondary and tertiary structures. Since denaturation reactions are not strong enough to break the peptide bonds, the primary structure (sequence of amino acids) remains the same after a denaturation process. Denaturation disrupts the normal alpha-helix and beta sheets in a protein and uncoils it into a random shape.

Protein in FoodsProtein in Foods

Protein in the diet can come from a Protein in the diet can come from a variety of sources. Some of the more variety of sources. Some of the more common ones are:common ones are:– EggsEggs– MeatMeat– FishFish– PoultryPoultry– legumeslegumes

EggsEggs

Eggs are a complex biological Eggs are a complex biological system, containing almost every system, containing almost every vitamin and mineral the body needs.vitamin and mineral the body needs.– Yolk: Rich in iron, phosphorus, vitamin Yolk: Rich in iron, phosphorus, vitamin

A and several B vitaminsA and several B vitamins The only vitamin lacking is Vitamin C.The only vitamin lacking is Vitamin C.

Although all these vitamins and Although all these vitamins and minerals are present, eggs are minerals are present, eggs are mainly considered a protein food.mainly considered a protein food.

StructureStructure

Albumen (Egg Albumen (Egg White)White)– Thick and thinThick and thin

YolkYolk Vitelline Vitelline

membranemembrane ChalazaChalaza Germ spotGerm spot Air pocketAir pocket

Proteins in EggsProteins in Eggs

Shell: Protein is interwoven with calcium Shell: Protein is interwoven with calcium carbonate. Shells are porous, so the carbonate. Shells are porous, so the developing chick can breathe.developing chick can breathe.

Albumen: Egg white, composes 54% of the Albumen: Egg white, composes 54% of the egg, and is named for ovalbumin, the egg, and is named for ovalbumin, the major protein in egg white.major protein in egg white.

Yolk: Contain the globular protein livetin, Yolk: Contain the globular protein livetin, and both high- and low- density lioproteins.and both high- and low- density lioproteins.– Thus, the yolk contains all of the fat, and most Thus, the yolk contains all of the fat, and most

of the other nutrients found in eggs.of the other nutrients found in eggs.

The ChalazaThe Chalaza

Chalaza: A twisted, ropelike Chalaza: A twisted, ropelike structure that keeps the egg yolk structure that keeps the egg yolk centered.centered.– Without it, the vitelline membrane could Without it, the vitelline membrane could

break, and the yolk (a fluid) would break, and the yolk (a fluid) would spread.spread.

Due to its thickness, the chalaza is Due to its thickness, the chalaza is the last part of the egg to coagulate, the last part of the egg to coagulate, and may remain slightly watery.and may remain slightly watery.

When the egg is freshly laid, the shell is completely filled. The air cell is formed by contraction of the contents during cooling and by the loss of moisture. A high-quality egg has only a small air cell.

The shell contains several thousand pores that permit the egg to "breathe."

Effects of Improper StorageEffects of Improper Storage

To retain their quality, do not store To retain their quality, do not store eggs in the egg tray of a refrigerator.eggs in the egg tray of a refrigerator.– It exposes eggs to light and temperature It exposes eggs to light and temperature

change every time the door is opened.change every time the door is opened. This speeds up chemical changes that cause This speeds up chemical changes that cause

physical changes that lead to deterioration.physical changes that lead to deterioration.

Loss of Egg QualityLoss of Egg Quality

Eggs lose quality as compounds in the egg Eggs lose quality as compounds in the egg white break down, forming water.white break down, forming water.– The white becomes less viscousThe white becomes less viscous

Some of the water enters the yolk, making it thinner, Some of the water enters the yolk, making it thinner, too.too.

Carbon dioxide and more water escape through the Carbon dioxide and more water escape through the porous shell.porous shell.

– This changes the pH from its normal 7.6 to over 9.This changes the pH from its normal 7.6 to over 9. Proteins begin to break downProteins begin to break down

Odors and flavors from other foods can Odors and flavors from other foods can enter through the shell, giving it an off tasteenter through the shell, giving it an off taste

Protecting EggsProtecting Eggs To delay quality loss, a thin film of natural To delay quality loss, a thin film of natural

oil in the shell protects the egg. oil in the shell protects the egg. – Shells may also be lightly coated with mineral Shells may also be lightly coated with mineral

oil immediately after the eggs are laid.oil immediately after the eggs are laid. Storing eggs closed in their carton helps Storing eggs closed in their carton helps

preserve their quality after purchase.preserve their quality after purchase. The best temperature for storing eggs is The best temperature for storing eggs is

-1 degree C.-1 degree C.This temp. is below the freezing point This temp. is below the freezing point

of water but well above the freezing point of water but well above the freezing point of the liquids found in eggs.of the liquids found in eggs.