Images shutterstock.com Chapter 11 Proteins: Amino Acids and Peptides

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Chapter 11

Proteins: Amino Acids and

Peptides

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

Objectives

• Identify amino acid classifications based on nutritional use and chemical properties of the side chains.

• Describe the primary, secondary, and tertiary structures of proteins.

• List at least six factors that denature proteins.

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Objectives

• State the functions of protein in food production.

• Apply basic principles of the chemistry of protein to cooking eggs, milk, and meat products.

• Compare the nutritional functions of proteins with the functions of carbohydrates and fats.


The Structure of Protein

• Protein is composed of carbon, hydrogen, oxygen, nitrogen, and usually sulfur– may also contain iron, copper,

phosphorus, or zinc

• Made up of subunits that are organic acids called amino acids– found in the human body, animals,

plants, and single-celled organisms

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• Amino acids have 3 basic parts– a side chain of carbon and hydrogen

atoms– a carboxyl group (-COOH)– an amine group (-NH2) of one nitrogen

atom and 2 hydrogen atoms

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• The carboxyl group acts as an acid and the amine group as a base

• When 2 amino acids combine, a peptide bond is formed and water is released

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• A polypeptide is a chain of amino acids bound together by peptide bonds– Most proteins have a chain from 100 to

500 amino acids

• Amino acids are classified by– their nutritional use– the chemical nature of their side chains

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• Classification by nutritional use– Out of the 20 amino acids needed by the

body for growth and body functions, many are dispensable (nonessential) amino acids that are made by the body

– Indispensable (essential) amino acids are the 9 amino acids that are not made by the body and must be supplied by the diet

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• Classification by nutritional use– Certain conditions prevent the body from

producing enough dispensable amino acids and they have to be obtained from the diet

– They become known as conditionally indispensable (conditionally essential) amino acids

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• Classification by nutritional use– Eggs, milk, fish, poultry, and meats are

called complete proteins because they contain all the indispensable amino acids

– Incomplete proteins from grains and vegetables are short one or more of the essential amino acids

– Combining incomplete proteins, such as red beans and rice, can form a complete source

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• Classification by side chains– The shape and function of protein

depend on the polarity of the side chains– Side chains can be nonpolar, uncharged

polar, positively charged, or negatively charged

– Nonpolar side chains are less soluble in water and are attracted to other nonpolar compounds, such as lipids and cholesterol

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• Classification by side chains– Amino acids with neutral polar side

chains will form hydrogen bonds and are attracted to other polar molecules, such as water

– Positively and negatively charged side chains enable some proteins to act as buffers


Protein Structures

• Proteins are complex molecules because of the– number of amino acids– order in which they combine– interaction of the side chains

• Primary structure is the order the amino acids occur in the chain– results from the chain of peptide bonds

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Protein Structures

• Secondary structure refers to the shape of sections of the amino acid– A helix is a repeating coil– A random coil is tangled and twisted– A pleated sheet is like a paper fan

• Tertiary structure refers to the 3-dimensional structure of an entire amino acid chain

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Protein Structures

• Tertiary Structure– Globular proteins do not form networks– Fibrous proteins

are usually made from helix-shaped strands, are strong and elastic, and form networks

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Molecular Interactions of Proteins• A hydrogen bond can form between

the hydrogen atom of one side chain and the hydroxyl group of another– Hydrogen bond formation is basic to the

stability of secondary and tertiary structures and makes some proteins water soluble

• Disulfide cross-links are covalent bonds between 2 protein molecules at side chains containing sulfur

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Molecular Interactions of Proteins• The more disulfide bonds there are,

the more stable the molecule is• Hydrophobic, or water repelling,

interactions occur between side chains that are nonpolar

• Hydrophobic proteins include– caseins in milk that form cheese curds– whey, a by-product of cheese production


Color Changes of Protein Pigments• Myoglobin is the iron-building

protein pigment in muscle that provides color– is bright red when oxygen molecule is

attached– is purplish if oxygen molecule is not

attached– is brown after prolonged exposure to

oxygen

• Oxidation and reduction is the reversible process of adding and removing oxygen

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Color Changes of Protein Pigments

• Nitrites are added during the curing process to preserve meats

• Cured meats such as ham and bacon– are very stable– have pink-red

color©Bochkarev Photography/Shutterstock.com


Denaturation of Proteins

• Any change in the shape of a protein molecule without breaking the peptide bonds is called denaturation– a loosening or unfolding of the tertiary

and sometimes secondary structure– is sometimes reversible– involves only hydrogen bonds

• Breaking disulfide cross-links, however, is not reversible

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• Coagulation is a permanent denaturation– results when liquid or semi-liquid proteins

form solid or semisoft clots

• The coagulation of a protein– changes its physical characteristics– alters the ability to bind with water– interferes with biological interactions of

enzymes

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• Some denatured proteins like beaten egg whites can return to their original state

• Coagulated proteins such as cooked eggs hold their new shape.

©Nattika/Shutterstock.com©kubais/Shutterstock.com


Physical Methods of Denaturing Protein• Temperature changes

– Heat speeds up denaturation– Colder temperatures may cause curdling

• Mechanical actions – Beating, rolling, and kneading disrupt

protein structures– Gluten strengthens during kneading

• Sound waves and irradiation– Prolonged exposure at high levels is

needed


Chemical Methods of Denaturing Protein• Change in pH

– Exposure to acids or alkalis can cause proteins to unfold

– Sour cream, buttermilk, and yogurt are the result of acids denaturing milk proteins

• Exposure to mineral salts or metals– Sodium and potassium salts and various

metals denature proteins


Functions of Protein in Food• To determine how effectively proteins

will work in a food product, food scientists analyze a protein’s– degree of water absorption– solubility– viscosity– stability in acids and alkalis

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Functions of Protein in Food1. Proteins form gels

– A protein gel is a mixture of fluids locked in a tangled 3-dimensional mesh of denatured and coagulated protein

– Gels have 2 parts—the 3-dimensional molecular structure and liquid that is attracted to the protein

– Protein gel has a narrow melting and solidifying temperature range

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Functions of Protein in Food• Acids soften gelatin, which may

develop syneresis if stored or cooked too long

• Gel stability – increases with mineral salts, hard water,

more gelatin, and slow cooling rates– decreases with acids, sugar, food pieces,

and rapid cooling

• A protein gel is formed from muscle tissue when salt is added to destabilize proteins

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Functions of Protein in Food2. Proteins can change texture

– Most globular proteins can be spun into fibers under the right conditions

• Texturizing is used– with soy protein to create meat

substitutes by denaturation or heat-coagulation under pressure

– to produce processed cheeses

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Functions of Protein in Food3. Proteins emulsify

– Denatured protein can act as an emulsifier since the polar side chain attracts water-based liquid and a nonpolar side chain attracts oil

– Pressure used in homogenization of milk enables casein to act as an emulsifier

– Protein’s ability to form an emulsion makes extraction of oils from seeds more difficult

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Functions of Protein in Food4. Proteins form foams

– A gas is suspended in liquid or a semi-solid

• Foams are formed by bubbling gas through a mixture, beating or whipping, and depressurization– Meringue, foam cakes, marshmallows,

souffles, and bread are examples– The albumin in egg whites and milk is a

good foaming agentcontinued


Functions of Protein in Food5. Proteins develop gluten

– Gluten is a strongly cohesive and elastic protein formed when a high-protein flour is combined with moisture and stirred or kneaded

– Strength results from the disulfide cross-links formed during kneading

– Trapped gases expand when heated, and the gluten structure coagulates when baked


Cooking High-Protein Foods• High temperatures and prolonged

cooking damage high-protein foods, which include– eggs– milk products– meat, poultry, and fish

• Protein molecules shrink and squeeze out water, resulting in a dry, rubbery, tough product


Storing and Cooking Eggs

• Eggs deteriorate when carbon dioxide is lost through the shell or water moves into the egg yolk

• Egg producers spray eggs to reduce loss of carbon dioxide and moisture

• Cooking eggs at low temperatures and short cooking times allow the egg white to coagulate, yet remain soft and tender


Principles of Cooking Milk

• Curdling in sauces and soups can be prevented by combining the acid with starch before adding milk

• Scorching of puddings and sauces can be prevented by using constant stirring, double boilers, and lower temperatures


Principles of Cooking Meat• Most meat contains muscle fibers,

connective tissue, and myoglobin– Goal is to soften collagen in connective

tissue while making muscle tissue more tender

• Moist heat methods work best on meat with much connective tissue

• Dry heat methods are ideal for meats with little connective tissue


The Maillard Reaction

• Amino acids in grains and meats at high temperatures have a chemical reaction that causes changes in food color and flavor– One step in the 3-phase reaction is

oxidation or dehydrogenation of an alcohol to form an aldehyde

• The Maillard reaction causes food to brown when cooked


Nutritional Contribution of Proteins• Support growth and repair

– Provide nitrogen and amino acids for growth and repair of muscle fibers, connective tissue, cell walls, and red and white blood cells

• Fight disease– Antibodies are proteins designed to

attack foreign substances

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Nutritional Contribution of Proteins• Maintain fluid and mineral balance

– Protein in cell walls control movement of fluids and minerals in and out of cells to help nerves, the brain, and muscles function properly

• Maintain pH balance– Proteins in the blood are buffers that pick

up or release acids and bases

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Nutritional Contribution of Proteins• Control bodily functions

– Proteins are a part of hormones that maintain body functions and enzymes that are necessary for chemical reactions in the body

• Provide energy– The body changes amino acids into an

energy source when extra protein is consumed

– The breakdown of excess protein produces ammonia and ketones, straining the kidneyscontinued


Nutritional Contribution of Proteins• Food scientists are developing

affordable proteins to meet future demands worldwide by– developing grains such as triticale and

amaranth– using biotechnology

• Health concerns– Food sensitivities and allergies require

accurate food labeling


• Food scientists are finding ways to meet future worldwide protein needs

• Protein molecules are made of subunits called amino acids consisting of– a side chain of carbon and hydrogen

atoms, a carboxyl group, and an amine group

• Protein structures include– primary, secondary, and tertiary

Recap

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• Proteins are denatured by– changing temperature– beating, rolling, and kneading– sound waves and irradiation at high

levels– changing pH

• Protein serves many functions in food production, including to– form gels, texturize, emulsify, form

foams, and develop gluten

Recap

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• High-protein foods must be carefully cooked to preserve moisture due to the rapid denaturation of protein when heated

• Nutritionally, protein functions to– support growth and repair, fight

disease, maintain fluid and mineral balance, maintain pH balance, control bodily functions, provide energy, and fight disease

Recap

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Images shutterstock.com Chapter 11 Proteins: Amino Acids and Peptides