1

Proteins (amino acids)

Normal body functions and health

Obesity

Human diseases

Chapter 6: Protein and Amino Acids

mm2k.com

A. The Chemist’s View of Proteins

• Proteins are made from 20 different amino acids, 9 of which are essential. 

• Each amino acid has an amino group, an acid group, a hydrogen atom, and a side group. 

-It is the side group that makes each amino acid unique:  size, shape, and electrical charge

• The sequence of amino acids in each protein determines its unique shape, function and complexity.

-carbonR

R =

= R

R = -CH2COOHR = CH3

The Chemist’s View of Proteins

• Amino Acids– Nonessential (dispensable) amino acids are ones the body can create, given the amino group and carbon back‐bone sources

– Essential (indispensable) amino acids must be supplied by the foods people consume. 

– Conditionally essential amino acids: nonessential but become essential under certain conditions• Tyrosine• Glutamine

The Chemist’s View of Proteins

• Proteins – Amino acid chains are linked by peptide bonds in condensation reactions.• Dipeptides: have two amino acids bonded together.• Tripeptides: 3 amino acids bonded together.• Polypeptides: >3 amino acids bonded together.  

– Amino acid sequences are all different, which allows for a wide variety of possible sequences.

TnTn

Text Box

Condensation rxn:

The Chemist’s View of Proteins

• Proteins– Protein Shapes:

• Dependent on the amino acid composition  > Hydrophilic side groups are attate to water.

> Hydrophobic side groups repel water.– Protein denaturation is the uncoiling of protein that changes its ability to function.• Proteins can be denatured by heat and acid.• After a certain point, denaturation cannot be reversed.

Human insulin Hemoglobin

Quaternary Structures

B. Digestion and Absorption

• Proteins in diets need to be digested and absorbed.

• In the Stomach • Protein is denatured by hydrochloric acid. • Pepsinogen (a proenzyme): converted into its

active form pepsin by hydrochloric acid. • Pepsin cleaves proteins into smaller

polypeptides.

Digestion and Absorption of Protein

• Protein Digestion – In the Small Intestine

• Proteases hydrolyze protein into short peptide chains (oligopeptides), which contain 4 to 9 AAs.

• Peptidases split tri- and dipeptides into AAs.

Protein Absorption – Transporters / carrier proteins (?) on membranes of

mucosal cells transport AAs and a few peptides into the cells of the intestine.

– Absorbed amino acids are • Used by intestinal cells: synthesis of proteins or

necessary compounds, energy • Excess is transported to the liver

– Taking enzyme supplements or consuming predigested proteins is unnecessary

Points to remember & understand:

•Proteins differ from other macronutrients: they contain nitrogen; and are more complex •Proteins are important because of the amino acids they contain •Amino acids can be designated as essential, non-essential or conditionally essential •Dietary proteins need to be digested and absorbed before our bodies can use them

C. Proteins in the Body

• Proteins are versatile and unique • The synthesis of protein is determined by genetic

information. • DNA (gene) • DNA is transcribed to messenger RNA, mRNA

(Transcription, in the Nucleus)

• Protein is constantly being broken down and

synthesized in the body (Protein Turnover). – Nitrogen Balance

• Synthesis • Breakdown (Proteolysis) • Excretion

• Protein Synthesis – Sequencing errors: altered proteins.

• e.g., sickle-cell anemia: incorrect AA sequence interferes with the cell’s ability to carry oxygen.

– Nutrients and Gene Expression - Cells regulate gene expression to make the type of protein needed for that cell.

– Regulation by nutrients – Protein Synthesis/Degradation? – Nutritional Regulation of Protein Turnover? – Nutritional Genomics and Proteomics?

Consequences of Defects in Protein Synthesis

Functions of Proteins

1)Building Materials for Growth and Maintenance • Collagen: filled with minerals to provide

strength to bones and teeth. • Replaces tissues including the skin, hair, nails,

and GI (gastrointestinal) tract lining 2)Enzymes: facilitate anabolic (building up) and

catabolic (breaking down) chemical reactions –Digestive enzymes –Enzyme in metabolism: e.g. formation of

glucose from AAS

3)Hormones regulate body processes and some hormones are proteins. • Blood glucose & Metabolism: Insulin & glucagon. • Metabolism: Thyroxin

Functions of Proteins in the Body 4)Regulators of Fluid Balance

• Plasma (?) proteins attract water, and maintain volume of body fluids to prevent edema

• Edema can happen because –Excessive protein losses caused by kidney

diseases, inflamation or large wounds –Inadequate protein synthesis caused by

liver disease –Inadequate protein intake

• Maintain the composition of body fluids

Blood

p. 182

Edema

Functions of Proteins

5)Acid-Base Regulators – Act as buffers by keeping solutions acidic or

alkaline • Acidosis: high acidity in blood & body fluids. • Alkalosis: high alkalinity in blood & body fluids.

6)Transporters (?) – Lipids, vitamins, minerals & oxygen in the body – Act as pumps in cell membranes, transferring

compounds from one side of the cell membrane to the other

Fig. 6-10a, p. 183

Key:

Cell membrane

Outside cell

Inside cell

Transport protein

The transport protein picks up sodium from inside the cell.

Sodium Potassium

Membrane transport

Functions of Proteins

7) Antibodies (immunoglobulins) –Fight antigens fragment of bacteria and viruses –Provide immunity to fight an antigen more

quickly the second time exposure occurs 8) Source of energy and glucose if needed • Other Roles

• Blood clotting: fibrin • Vision: Opsin in the retina

D. A Preview of Protein Metabolism – Protein Turnover and the Amino Acid Pool

• Continual making & breaking down of protein. • Pattern of AA pool is fairly constant,

irrespective of intake and/ or rate of protein breakdown.

– Exogenous amino acids – Endogenous amino acids

– Nitrogen Balance: balance of protein intake and output • used to estimate protein requirement

• Protein turnover & nitrogen balance go hand in hand.

– Zero nitrogen balance is nitrogen equilibrium, • N intake = N output in urine • Protein synthesis = protein breakdown

– Positive nitrogen balance: nitrogen consumed> nitrogen excreted. • Growing infants, children, adolescents, pregnant

women, people recovering from protein deficiency or illness

– Negative nitrogen balance: nitrogen excreted > than nitrogen consumed • Burns, injuries, infections, fever • Protein synthesis < protein breakdown

Proteins in the Body • A Preview of Protein Metabolism

– Amino Acids • Make Proteins or nonessential AAs (?) • Make other compounds

–Tyrosine: Neurotransmitters epinephrine and norephinephrine.

–Tyrosine: melanin pigment & metabolic hormone thyroxine.

–Tryptophan: »Niacin, and serotonin (neurotransmitter).

• Energy and Glucose: no storage of proteins

Proteins in the Body • A Preview of Protein Metabolism

– Deaminating Amino Acids • Nitrogen-containing amino groups are removed. >Ammonia is released into the bloodstream.

–Ammonia is converted into urea by the liver. –Kidneys filter urea out of the blood.

>Carbon backbone is used in other metabolic pathways: glucose, fat

– Using Amino Acids to Make Fat • Excess protein is deaminated and converted

into fat; Nitrogen (NH3) is excreted.

Deamination of AAs

Transamination & synthesis of non-essential amino acids

Can essential amino acids be made by this process? What will happen if dietary supply of EAA is not adequate?

Preview of Protein Metabolism: Urea synthesis

Fig. 6-14, p. 187

Bloodstream

To bladder and out of body

Kidney

Urea

Bloodstream

Urea

Liver

Ammonia (NH3) +

CO2

Amino acids

Urea

Liver disease: high ammonia

Kidney disease: high urea

Preview of Protein Metabolism

Excreting urea Liver releases urea into blood Kidneys filter urea out of blood

Liver disease Kidney disease Protein intake and urea production Water consumption Implication for weight loss diets

Points to remember & understand:

• Link between genetic information and protein synthesis – DNA, gene, mRNA, tRNA, rRNA – Consequences of ‘mistakes’ in the process

• Functions of proteins – Protein turnover and nitrogen balance

• Fates of amino acids: deamination, transamination, keto acids, ammonia, urea

E. Protein in Foods • Protein Quality: All proteins are not the same

1) Digestibility • Depends on protein’s food source

–Animal proteins are 90-99% absorbed. –Plant proteins are 70-90% absorbed. > Soy and legumes are 90% absorbed.

• Other foods consumed at the same time can affect digestibility

Protein in Foods: protein quality

2) Amino Acid Composition • The liver can produce nonessential AAs. • Cells must dismantle to produce essential AAs

if they are not provided in the diet. • Limiting amino acids (?)

– Reference Protein is the standard by which other proteins are measured. • Based on their needs for growth and

development, preschool children are used to establish this standard.

Protein in Foods Protein Quality

– High-Quality Proteins • Contains all the essential AAs >Animal foods contain all the essential AAs. >Plant foods: diverse in content & tend to be

missing one or more essential AAs. – Complementary Proteins: combining plant foods

that together contain all the essential AAs • Used by vegetarians

Assessing Protein Quality • Scarcity of proteins in some regions of the world

– not in Canada • Measures of Protein Quality (Appendix D)

– 1) Amino Acid scoring – 2) Protein digestibility-corrected amino acid

score (PDCAAS) – 3) Biological Value (BV) – 4) Net Protein Utilization (NPU) – 5) Protein Efficiency Ratio (PER)

Protein Regulation for Food Labels

– Food labels must list protein quantity in grams – % Daily Values is not required but reflects

quantity and quality of protein using PDCAAS.

F. Health Effects and Recommended Intakes of Protein

• Protein deficiencies arise from – protein-deficient diets and – energy-deficient diets.

• Worldwide malnutrition problem, especially for young children. Other groups?

• High-protein diets have been implicated in several chronic diseases

• Protein-Energy Malnutrition (PEM); also called protein-kcalorie malnutrition (PCM)

• Chronic PEM: the child is short for his/ her age and

• Acute PEM: child is thin for his/her height • Maramus, kwashiorkor, or a combination of

the two

PEM

Copyright © 2013 by Nelson Education Ltd.

Table 6.3, p. 187

Health Effects & Recommended Intakes of Protein

• PEM – 1. Marasmus: – Chronic PEM

‘Little old people’

– 2. Kwashiorkor: Acute PEM • Older infants and young children,

18 months to 2 years of age

Health Effects and Recommended Intakes of Protein

• PEM – Infections

• Lack of antibodies to fight infections • Fever • Fluid imbalances and dysentery • Anemia • Heart failure and possible death

–Dysentery, measles worsen condition

• Rehabilitation of PEM: – Nutrition intervention must be cautious, slowly

increasing protein. • Programs involving local people work

better.

Health Effects and Recommended Intakes of Protein

• Negative Health Effects of Protein – Limitations – Heart Disease

• Foods high in animal protein also tend to be high in saturated fat.

• Homocysteine levels increase cardiac risks.

• Arginine may protect against cardiac risks.

Health Effects and Recommended Intakes of Protein

• Health Effects of Protein – Cancers

• High protein intake vs. high fat intake – Adult Bone Loss (Osteoporosis)

• High protein intake associated with increased calcium excretion.

–Animal vs. vegetable proteins • BUT: Inadequate protein intake affects bone

health

– Weight Control • High-protein foods are often high-fat

foods. • Protein at each meal provides satiety.

– Kidney Disease • Does not seem to cause kidney disease

Recommended Intakes of Protein • Daily protein intake

– Supply of essential AAs – Source of N for synthesis of non-essential AAs

• Recommended Intakes of Protein – 10-35% energy intake (AMDR) – Protein RDA: 0.8 g/kg/day; athletes: 1.2-

1.7g/kg/day • Assumptions

–People are healthy; –Protein is of mixed quality. –The body will use protein efficiently.

p. 194

Health Effects and Recommended Intakes of Protein

• Recommended Intakes of Protein – Adequate Energy

• Must consider energy intake • Must consider total grams of protein

– Protein in abundance is common in the U.S. and Canada.

Protein and Amino Acid Supplements

Protein powders Muscle work vs. protein supplements Athletic performance

Whey protein Impact on kidneys

Amino acid supplements Potential risks associated with intake Lysine: herpes Tryptophan: pain and sleep

Nutritional Genomics

Nutrigenomics Nutrients influence

gene activity Nutrigenetics Genes influence

activity of nutrients Human genome

Fig. H6-3, p. 201