10252021 By Shu-Chen Hsieh

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Transcript of 10252021 By Shu-Chen Hsieh

10252021 By Shu-Chen Hsieh

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Institute of Food science and technology

Professor 謝淑貞

scjhsieh@ntu.edu.tw

TEL:33669871

食科大樓 R217

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➢Physicochemical properties of amino

acids

➢Chemical reactivity of amino acids

➢Processing induced changes in

proteins

➢Enzymatic modification of proteins

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Physicochemical properties of

amino acids

Structure of amino acids Carboxyl group

Amino

group

Hydrogen

atom

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Glycine

Aliphatic amino acids

(Val) (Leu) (Ile)

(Gly)[G]

[V] [L] [I]

CH3

Alanine (A)

(Ala) 6

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Aromatic amino acids

(Phe) (Tyr) (Trp)[F] [Y] [W]

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Acidic amino acids and their

amides

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Basic amino acids

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Sulfhydryl amino acids

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Formation of cystine

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The heterocyclic pyrrolidine ring restricts the geometry of polypeptides

Cyclic amino acids

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(Ser) [S] (Thr) [T]

Hydroxyl amino acids

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Derived natural amino acids

O-phosphoserine (casein) 4-hydroxylysine (collagen)

N-Methyllysine (myosin) 4-methylproline

(apple)

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Derived AAs from processing

From heated milk15

◼Aspartame is a

dipeptide methyl ester

(aspartylphenylalanine

methyl ester), an

artificial sweetener

◼About 200 times

sweeter than table

sugar

◼Used in diet drinks

Aspartame

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Stereochemistry of amino acids

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Chiral center

Solid wedges: bonds

coming forward

Shaded wedges: receding

bonds

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◼19 of the 20 common amino acids have a chiral α-

carbon atom (exception: Gly)

◼Mirror image pairs of amino acids are designated

L (levo) and D (dextro)

◼Proteins are assembled from L-amino acids

(a few D-amino acids occur in nature)

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◼Amino acid “residues” compose peptide chains

◼Peptide chains are numbered from the N (amino)

terminus to the C (carboxyl) terminus

◼Example: (N) Gly-Arg-Phe-Ala-Lys (C)

(or GRFAK)

◼Formation of peptide bonds eliminates the ionizable

a-carboxyl and a-amino groups of the free amino

acids

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◼Nonpolar: aliphatic, aromatic

◼Polar non charged: hydroxyl, sulfhydryl

◼Polar charged: acidic, basic

Polar-hydrophilic

Nonpolar-hydrophobic

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Acid-base properties of amino acids

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Henderson-Hasselbach equation

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◼Under normal cellular conditions amino acids

are zwitterions (dipolar ions):

Amino group = -NH3+

Carboxyl group = -COO-

◼A zwitterion is a molecule with

equal numbers of positive and

negative charges - thus the net

charge is zero

◼An ampholyte is a molecule containing

groups with both acidic and basic pKa

values

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pI

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pKa1pKa2

pKa326

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H+

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◼Hydrophobicity can be defined as the excess energy of a

solute dissolved in water compared to that in an organic

solvent under similar conditions

◼ The most direct and simplest way to estimate

hydrophobicity of AA side chain is experimental

determination of free energy changes for dissolution of

AAs side chains in water and in an organic solvent, such

as octanol or ethanol.

◼AAs side chains with large positive free energy

(oct→water ) are hydrophobic; they would be prefer to be

in an organic phase rather than in an aqueous phase.

Hydrophobicity of amino acids

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Residue area (Å 2)

0 50 100 150 200 300250

The hydrophobicity of nonpolar residues

is linearly correlated to their surface area

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Optical properties of amino acids

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Reaction with ninhydrin

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Chemical reactivity of amino

acids

Reaction with O-phthaldialdehyde

Emission 450 nm

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Reaction with fluorescamine

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◼Food processing and storage can either

transduce benefic or detrimental effects on food

◼Detrimental effects mainly come from

➢ Loss of essential amino acids

➢Toxic substances formation

Processing induced changes in proteins

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Chemical processing

◼Heating

◼Cooling

◼Drying

◼Application of chemicals

◼Irradiation

◼Fermentation

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High temperature heat treatment

>200oC

◼Destruction of amino acids: eg.

RSTK

◼Mutagenic compounds generation:

aminoimidazoazaarene

Heterocyclic amines39

◼Improving digestibility

◼Inactivating enzyme eg lipoxygenase

◼Inactivating inhibitors of digestive

enzymes eg ovomucoid

◼Destroying toxins eg enterotoxin from

Staphylococcus aureaus

◼Inactivating antinutritional factor eg lectin

60‐90oC, 1h, partial denaturation

Moderate heat treatment

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PER: protein efficiency ratio

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紅肉遭WHO列為致癌物業者跳腳(20151027 法國世界報)

IARC的主任威爾德博士表示,紅肉雖有營養價值,人們仍應避免過多攝取。該機構的統計表明,常人每日一旦多攝取100公克的紅肉,大腸癌的風險就高出17%;如果是加工肉品,更只要50公克就會高出18%。目前全球癌症的死亡人口中,每年有5萬人可以咎因於紅肉飲食,而這又大都集中於富裕國家。所謂的紅肉,包括:牛肉、小牛肉、豬肉、羔羊、山羊肉、馬肉等。加工肉品則指使用過鹽漬、催熟、發酵、燻烤等各種製程的肉類。尤其是高溫烹調時肉品表面產生的多環芳烴和雜環胺,生肉中的血紅素鐵、臘肉使用的硝酸鹽/亞硝酸鹽,彼等的致癌性都讓科學家不敢輕忽。

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IARC: international agency for research on cancer

Racemization

◼High [OH] > Low[OH]

◼protein > AAs

◼Preference for DSCFENT

◼Decreased digestibility

◼Generation of toxic products eg

D-proline

Cysteine

phosphoserine

Chemical alteration of amino acids

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Compositional changes during

extraction and fractionation

◼Isoelectric precipitation-lost of the Met, Trp in

coconut protein isolates

◼Ultrafiltration and ion exchange-lost of

foaming property of whey protein

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Protein cross-linking (1/2)

1000X faster than lysine

Cysteine

Ammonia

sulfites

◼Decreased the

digestibility

◼Reduce the biological

availability of essential

amino acids

Lysinoalanine Lanthionine45

Excessive heating of

pure protein or

proteinaceous food

low in carbohydrate

content

◼Decreased digestibility

◼Decreased bioavailability

Protein cross-linking (2/2)

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Ionizing radiation

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➢Bactericidal agents

➢Detoxification

➢Bleaching

◼The use of oxidizing agents in foods

◼Oxidative compounds produced in processing

➢Free radicals

➢Peroxidation of lipids

➢Photooxidation

➢Nonenzymatic browning of foods

Effects of oxidizing agents

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Oxidation of methionine

Biological unavailable

Strong oxidizing conditions

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Oxidation of cysteine and cystine

Biological unavailable

Biological available

Biological unavailable

Biological available

At acidic pH

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Oxidation of Tryptophan

Severe oxidizing

condition

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Photooxidation of Tryptophan

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Occurrence of this type of cross-link has been found in natural proteins ,

such as elastin, keratin, and collagen, and more recently in dough

Oxidation of Tyrosine

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Maillard reactionReaction between amines and carbonyl compounds (C=O)

at elevated temperature, which will decompose and

eventually condense into insoluble brown products known

as melanoidins

Carbonyl-amine reactions

◼Loss of lysine

◼Oxidation of some amino acids. WHYM

◼Decrease of protein solubility

◼Lower digestibility

◼Mutagens formation eg products of CK

◼Amino reductones: antioxidative activity

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Schiff’s base

Strecker degradation

Distinctive aroma

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Gossypol

Glutaraldehyde

aldehydes

◼Insolubilization

◼Loss of digestibility and bioavailability of lysine

◼Loss of functional properties of proteins

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Acrylamide

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AGE (advanced glycation end-product)

AGEs are formed both outside and inside the body.

Specifically, they stem from glycation reaction, which

refers to the addition of a carbohydrate to a protein

without the involvement of an enzyme.

In humans, histones in the cell nucleus are richest in

lysine, and therefore form the glycated protein N(6)-

Carboxymethyllysine (CML)

AGEs affect nearly every type of cell and molecule in the

body and are thought to be one factor in aging and some

age-related chronic diseases

Dietary AGE can form in food during cooking, particularly

in dry cooking such as frying, roasting and baking, far less

so in boiling, stewing, steaming and microwaving. 59

Reaction with lipids

Oxidation of unsaturated lipids leads to formation of alkoxy and peroxy

free radicals. These free radicals in turn react with proteins, forming a

variety of cross-linked products

◼Decreased nutritional value

◼Off flavor60

Reaction with Halogenated solvent

◼ Halogenated organic solvent are often used to extract

oil and some antinutritive factors from oil seed products

◼ The products of the reaction between protein and

halogenated might be toxic eg extraction with

trichloroethylene results in the formation of small

amount of S-dichlorovinyl-L-cysteine

◼ Might cause the alkylation of KHC eg methyl bromide,

thus decrease the nutritional value

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CK Highly reactive

Reaction with

polyphenol

HMW: High molecular weight

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Reaction with Nitrites

Formation of N-nitrosoamines can be decreased by ascorbic acid and

erythorbate

Improve color

Prevent bacteria growth63

◼Bioavailability of cysteine is not altered

◼Unfolding of protein may happen

Reaction with sulfites

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Enzymatic hydrolysis

Partial hydrolysis of proteins using proteolytic enzymes is one

of the strategies for improving the functional properties. Protein

hydrolystes have many uses in specialty food such as geriatric

foods, nonallergenic infant formula, sports drinks, and diet

foods

Enzymatic modification of proteins

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◼Food protein with allergenicity

Cow’s milk

➢Casein

➢Lactoglobulin

➢lactalbumin

Soy proteins

Gluten

Egg proteins

Peanut proteins

◼Hydrolysis of casein up to 55% DH (degree of

hydrolysis) decreases its allergenicity by about 50% ARI

(allergenicity reduction index)

Hydrolysates make proteins less allergic

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◼Peptides with a mean residue hydrophobicity of

>1.4 Kcal/mol are bitter.

◼Hydolysates of highly hydrophobic proteins

(casein, soy protein, and corn protein)-very bitter.

◼Hydolysates of highly hydrophilic proteins

(gelatin)-less bitter.

Solutions?

The bitterness emanates from certain peptides

released during hydrolysis

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Plastein reaction

◼The plastein reaction refers to a set of reactions

involving initial proteolysis, followed by

resynthesis of peptide bonds by a protease.

◼The enzyme, especially papain and

chymotrypsin, acts both as a protease and an

esterase under certain conditions.

◼The plastein reaction can be exploited to

improve the nutritional quality.

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Protein cross-linking

Transglutaminase

◼Improve nutritional quality71

◼Physicochemical properties of Amino acids✓Structure of amino acids✓Stereochemistry of amino acids✓Acid-base properties of amino acids✓Hydrophobicity of amino acids✓Optical properties of amino acids◼Chemical reactivity of amino acid◼Processing induced changes in proteins◼Enzymatic modification of proteins

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