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Protein & Amino Acids

Readings: Chapter 6

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Outline• What are Proteins?• Amino Acids (essential AA)• Protein structure • Digestion and Absorption• Protein Synthesis in the cell• Functions of Proteins in the body

– Structure, enzymes, hormones, immunity, fluid and acid balance, transport and energy

• Protein turnover and nitrogen balance• Recommended intake• Protein Quality• Protein malnutrition• Too much Protein?• Vegetarian Diets

– Complementary proteins

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Introduction• proteins are the building blocks of the

human body, forming about 17% of body weight

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What are Proteins?• organic compounds• contain carbon, hydrogen, oxygen and

nitrogen atoms• formed from “building blocks” called

amino acids

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Amino Acids• building blocks of proteins• have 5 components:

– central carbon atom (C)– a hydrogen atom (H)– an acid group (COOH)– an amino group (NH2)– a side chain

• the side chain for each type of amino acid is UNIQUE• there are 20 common amino acids

H C

H

N

H

HC

O

O

Figure 6-1 page 168

side chain

acid group

amino group

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Essential Amino Acids• nine of the amino acids are essential and

must be obtained in the diet (text refers to them as indispensable)

• the non-essential amino acids can be synthesized by the body

• in certain conditions, some non-essential amino acids may become conditionally essential

histadineisoleucineleucine

lysinemethioninephenylalanine

threoninetryptophanvaline

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• amino acids are linked together by peptide bonds to form peptides and proteins

• occurs via a condensation reaction • three amino acids = a tripeptide• many amino acids = a polypeptide

Amino Acids (A.A.)

= a DIPEPTIDE

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Protein Structure: A.A. Sequence

• the primary structure of a protein refers to its sequence of amino acids

• similar to the letters of the alphabet used to make words, sentences, poems, novels

• form an almost infinite variety of sequences to make an estimated 100,00 - 500,000 different proteins in the human body

Figure 6-4 page 170

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Protein Shape• proteins have complex three dimensional

shapes depending on the amino acid sequence– secondary structure – tertiary structure – quaternary structure

• these unique structures are important for the protein to function properly in the body

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Protein Shape

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Protein Shape

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Protein Shape

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Protein DigestionMouth• chewing• no enzymatic digestionStomach• hydrochloric acid (HCl) denatures the protein

– protein loses structure and shape– Also occurs with exposure to alkalinity,

agitation, alcohol or cooking

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Stomach cont.• hydrochloric acid (HCl) also activates the

enzyme pepsin

• enzyme cleaves large polypeptides into smaller polypeptides and amino acids

• pepsin is responsible for 10 - 20% of protein digestion

Protein Digestion

pepsinogen pepsinHCl

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Protein DigestionSmall Intestine• site of most protein digestion• pancreatic and intestinal cells secrete enzyme

precursors called proenzymes– when proenzymes reach the lumen of small intestine,

part of proenzyme is cleaved off, activating the enzyme – this is important because it protects the cells that

secrete the enzymes from being digested• enzymes that digest proteins are called

proteases

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Protein DigestionSmall Intestine cont.• an enzyme secreted by the intestinal cells called

enteropeptidase activates the enzyme trypsin

• trypsin digests polypeptides and activates other enzymes

• dipeptidases & tripeptidases digest dipeptides & tripeptides into single a.a.’s

trypsinogen trypsinenteropeptidase

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Protein Absorption• requires specific transporters for individual

amino acids, dipeptides, and tripeptides– some absorbed by active transport– others absorbed by facilitated diffusion

• some amino acids compete for the same transporter

• intestinal cells keep the amino acids they need

• the rest enter the blood and travel to the liver

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Protein Synthesis• the sequences in which amino acids are joined

to form proteins is determined by our genes• genes are small segments of DNA, which

serves as a template for protein synthesis• DNA is composed of two strands of linked

nucleotide chains that are twisted together to form a double helix

• like a code, the sequence of nucleotides determines the order in which the amino acids are combined

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Protein Synthesis• if a non-essential amino acid is missing

during protein synthesis, the cell may either:- make the amino acid, or- obtain it from the liver

• if an essential amino acid is missing during protein synthesis:- the body may break down another body protein to

provide the essential amino acid- if the essential amino acid is not available,

protein synthesis will stop

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Protein SynthesisGenetic disorders result from either:• an error in the original DNA copy• errors in translating the DNA sequence into

the corresponding amino acid sequenceExample: sickle cell anemia• due to a genetic mutation in the protein

hemoglobin which carries O2 in the blood• hemoglobin is unable to carry oxygen

efficiently, results in many medical problems

Structure

Enzymes

Hormones

AntibodiesFluid BalanceAcid-Base

Balance

Membrane Transporters

Transport in the blood

Synthesis of non-protein compounds

Functions of ProteinsFunctions of Proteins

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Structure & Mechanical Functions

• building blocks of most body structures

• important for growth and development

• also needed for REPAIR & MAINTENANCE of tissues

• examples: collagen, keratin, motor proteins

25Figure 6.7, pages 212 & 213

Enzymes• acts as catalysts to facilitate chemical

reactions• change other molecules while remaining

unchanged themselvesExample: digestive enzymes

Figure 6-9, page 177

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Hormones• hormones are

messenger molecules that travel through the blood

• regulate many body processes

Examples: insulin, glucagon

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Immune Functions• form antibodies that help fight off infectious

disease• synthesized by immune cells called

lymphocytes in response to an infection• label the pathogen so that other immune cells

can recognize the pathogen and destroy it

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Regulation of Fluid Balance• all compartments within the body are

filled with fluid• proteins are required to balance fluid

distribution between these compartments

CELL

INTERSTITIAL SPACE

CAPILLARY

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Regulation of Fluid Balance• pressure of blood against the walls of

the capillary tends to push fluid out

INTERSTITIAL SPACE

BLOOD

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Regulation of Fluid Balance• pressure of proteins tends to pull fluid in

INTERSTITIAL SPACE

PROTEINS

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Regulation of Fluid Balance• a in plasma proteins ’s the forces

holding fluid in the blood & fluid leaks into the interstitial space causing edema

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Regulation of Fluid Balance

Occurs with inadequate dietary protein, kidney disease, burns, & liver disease.

Edema of the leg

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Acid-Base Balance• the acidity of a solution is determined by the

amount of hydrogen ions it contains• measured on a logarithmic scale called the pH

scale

Acids: release H+ into a solutionBases: accept H+ from a solution

• proteins accept & release H+ to regulate pH

acidity pH acidity pHALKALOSIS ACIDOSIS

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Transport Functionsa) transport molecules in the

bloodExamples:

– lipoproteins– hemoglobin– transferrin

b) transport molecules across membranes

Examples– sodium - potassium pump

(Na+/K+ pump)

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Source of Energy and Glucose

• provides 4 kcal per gram• normally does NOT contribute much to

energy supply of bodyConditions in which protein is used for energy:

– consumed in excess of body’s needs to synthesize body proteins

– dietary intake of carbohydrate is insufficient to supply glucose

– athletic endurance events

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Source of Energy & Glucose• in order to use proteins as energy, the amino

group (NH3) must be removed– process is called deamination

• produces:– KETO ACID: excess can disrupt acid-base

balance– AMONIA: converted to UREA by the LIVER,

then excreted by the KIDNEY• remaining components (carbon skeleton) used to

provide energy or synthesize glucose

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A.A. Pool & Protein Turnover

Amino Acid Pool= amino acids free in body

tissues & fluids available for new protein synthesis

Protein Turnover= the constant synthesis &

breakdown of proteins in the body

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Protein and Nitrogen Balance• cells are constantly turning over protein• balance between protein degradation and

synthesis can be determined by measuring:– the amount of nitrogen in protein consumed– the amount of nitrogen excreted from the body

Nin = Nout NITROGEN EQUILIBRIUM

• in healthy adults, protein synthesis is balanced with degradation

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Protein and Nitrogen BalanceNin > Nout positive nitrogen balance• if the body synthesizes more protein than it

degrades, positive nitrogen status occurs• when does this occur?Nin < Nout negative nitrogen balance• body degrades more than it synthesizes

resulting in a net protein loss from the body• when does this occur?

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Food Sources of Protein

meats

fish & seafood

poultry

LEGUMES NUTS & SEEDS

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Recommended Intakes• recall: DRI are meant for healthy people• recommendations assume protein is

consumed with adequate carbohydrate & fat to provide a healthy energy intake

• typical NA intake is 95 g/day (men) & 65 g/day (women)

• AMDR = 10-35% of caloric intake generally overestimates needs

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Recommended IntakesHealthy Adults

Pregnancy

Power Athletes

Endurance Athletes

0.8 g/kg/day

0.8 g/kg/day + 25 g

1.6 - 1.7 g/kg/day

1.2 - 1.6 g/kg/day

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Proteins in the Diet: Sources of Protein

Example:• male weighing 160 pounds (72.7 kg)

needs 58 grams of protein per daychicken drumstick 12 g1 egg 6 g¼ cups peanuts 9.5 g½ cup chopped broccoli 3 g½ cup lentils 9 g1 bagel 7 g1 banana 1 g1 cup skim milk 9 g½ cup tuna 19.5 g

Breakfast14 g

Lunch18 g

Which supper would you choose to provide sufficient protein without exceeding the RDA for an individual who weighs 132 pounds (60 kg)?RDA = 0.8 g/kg x 60 =

48 g

27 g 18 g 51 g

= 32 g

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Protein Quality• determined by the protein’s ability to supply the essential amino acids and how well it is

used in the bodyComplete Proteins• contain all the essential amino acids in relatively the same amounts as the body requiresExamples:• most animal derived proteins• SOY PROTEIN

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Protein QualityIncomplete Proteins• fail to provide all of the essential amino

acids in sufficient quantities to support the bodies needs

Examples: vegetables, legumes, grains

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Protein QualityLimiting Amino Acid• the essential amino acid found in the LEAST

amount relative to the amounts needed to support protein synthesis in the body

Examples:– legumes often have insufficient methionine

and tryptophan– grains commonly have insufficient isoleucine

and lysine

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Protein QualityWhat happens if the diet is lacking one or more of the essential amino acids?

PROTEIN SYNTHESIS WILL BE LIMITED!

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Protein QualityDigestibility• a measure of the amount of amino acids

absorbed from a given protein intake• influenced by:

– the proteins food source– other foods consumed with it

• PLANT PROTEINS are LESS digestible because they are contained within cell walls that resist digestion

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Protein QualityReference Proteins• used as a standard for measuring protein quality, assigned a

value of 100Chemical or Amino Acid Scoring• compares a protein’s aa pattern with a reference proteinProtein Efficiency Ratio (PER)• measures growth of an animal (rats) compared to the anima

l’s protein intakeNet Protein Utilization (NPU) / Biological Value• measures how efficient a protein is at supporting the body’s

needsPDCAAS• calculate amino acid score• multiply by the digestibility

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Protein Energy Malnutrition (PEM)

• results when an individual is deprived of PROTEIN or ENERGY or BOTH Protein AND Energy

• more common in children• most prevalent form of malnutrition

worldwide, affecting over 500 million children• associated with infections, diarrhea, and

multiple nutrient deficiencies• two forms: marasmus & kwashiorkor

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Marasmus • “dying away”• chronic protein AND energy

deficiency• most common in children 6 - 18

mos.• “skin and bones” appearance• muscle & tissue wasting• cessation of growth• impaired brain development &

learning ability• increased incidence of infections

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Kwashiorkor • develops rapidly (acute)• severe protein deficiency, diet still

provides limited energy• occurs between 18 - 24 mos.• characterized by edema in face

and limbs & a bloated belly• development of a fatty liver• increased incidence of infections• also see loss of appetite, apathy,

skin lesions

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Too much Protein?

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Protein SupplementsBuild Muscle?• physical training builds muscle requiring

some extra protein• recommendation for power athletes is 1.6 - 1.7

grams of protein per kilogram of body weight• exercise does not require protein to provide

energy• beyond 2 g/kg, studies show no increased

synthesis of muscle protein

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Amino Acid SupplementsImprove Health?• body not designed to handle high

concentration (increases demand for transporter), may cause deficiency of another, potentially dangerous

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Vegetarian DietsTypes of Vegetarians:- Vegan- Lacto-Vegetarian

– (“lacto” = milk)

- Lacto-Ovo-Vegetarian – (“ovo” = egg)

- Lacto-Ovo-Pesca-Vegetarian– (“pesca” = fish)

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Vegetarian Diets: Health Benefits?

• healthier body weight• lower blood pressure risk of coronary artery disease incidence of cancer

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Protein in a Plant-Based Diet• in general plant derived proteins are lower

quality than animal derived proteins• need to ensure adequacy of the essential amino

acids• dairy and eggs provide high quality proteins for

lacto-ovo-vegetarians• employing the dietary planning principle of

variety ensures vegans are not lacking• involves combining plant protein foods that have

different but complementary amino acid patterns

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Protein in a Plant-Based DietComplementary Proteins• two or more proteins that make up for

each others inadequate supply of specific essential amino acids (i.e. the LIMITING AMINO ACIDS) so that together they yield a sufficient amount of all nine essential amino acids

Ile Lys Met Trplegumesgrainstogether

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Protein in a Plant-Based Diet

Mutual Supplementation• the dietary strategy that brings

complementary proteins together so that each food supplies the amino acids lacking in the other

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Health Risks of Vegetarianism?• a poorly planned vegetarian diet can

potentially lead to several nutrient deficiencies:

• Iron• Zinc• Calcium• Vitamin B12

• Vitamin B6

• Vitamin D• remember that a vegetarian diet is also not

necessarily low in fat & high in fiber!

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Dietary Recommendations for Vegetarians

• choose a variety of foods, including whole grains, vegetables, fruits, legumes, nuts, seeds, and if desired, dairy products and eggs

• choose whole, unrefined foods often; minimize intake of highly sweetened, fatty, and heavily refined foods

• if dairy products and eggs are consumed, limit consumption of high fat dairy foods and eggs

• vegans should consume a regular source of Vitamin B12 (and if sun exposure is limited, Vitamin D also)