Ch 5 ppt

CHAPTER 5

Proteins

Eleanor D. Schlenker

Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

2 Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

Physical and Chemical Nature

of Protein

In 1838 Dutch chemist Johann Mulder first

identified protein as a prime substance in all

life forms


Physical and Chemical Nature

of Protein – Cont’d

Protein enzymes break down our food

Antibodies shield us from disease

Peptide hormones carry messages

Structural proteins in our bodies


The Nature of Amino Acids

Amino acids are composed of the elements

carbon, hydrogen, oxygen, and nitrogen

Several contain sulfur

Amino acids are the building blocks for body

tissues

Word amino refers to a base or alkaline

substance


General Pattern and Structure

Common structural pattern holds for all amino

acids

This pattern is built around a central alpha-

carbon with several attached chemical

groups:

Amino (base, NH2) group

Carboxyl (acid, COOH) group

Attached radical (R) group


Essential Amino Acids

20 amino acids are used to build body

proteins

Indispensable (essential) amino acids: 9 that

cannot be synthesized by the body and must be

supplied in food

Dispensable (nonessential) amino acids: 5 that

can be synthesized by the body in adequate

amounts

Conditionally indispensable amino acids: 6 that fall

in between; the body is unable to synthesize the

amount needed under certain conditions


The Building of Protein

Ability of amino acids to form peptide linkages

and arrange themselves into peptide chains

is basic to all proteins and body tissues

Peptide bonds: the joining of the end amino

group of one amino acid with the end

carboxyl group of the amino acid beside it


The Building of Proteins – Cont’d

Polypeptides: specific amino acids joined in a

particular sequence to form long chains of

amino acids

Oligopeptides: short chains of 3 to 15 amino

acids


Types of Protein

Myosin: fibrous protein found in muscle fiber,

built from chains of 153 amino acids that coil

and unfold as the muscle contracts and

relaxes

Collagen: contains three separate

polypeptide chains that wind around each

other to produce a triple helix


Types of Protein – Cont’d

Hemoglobin: globular type of protein that

includes four globin polypeptide chains per

molecule of hemoglobin

Albumin: major plasma protein that has a

compact globular shape, consists of a single

polypeptide chain of 584 amino acids


Functions of Protein

Dietary protein supplies building material for

the growth and maintenance of body tissues

Dietary protein furnishes amino acids in the

appropriate patterns and amounts for efficient

synthesis of specific structural molecules


Physiologic Roles

All amino acids participate in tissue growth

and maintenance


Physiologic Roles – Cont’d

Some have important metabolic roles of their

own:

Form neurotransmitters for brain and nerve

function

Form other amino acids

Form hormones

Support immune function

Maintain fluid balance


Role of Protein in Critical Care

Certain amino acids appear to have a

particular role in recovery from severe

infection, trauma, or life-threatening

malnutrition

Recovery in these situations may be

improved with specific amino acid

supplements


Role of Protein in Critical Care –

Cont’d

Leucine: following surgery and trauma

Branched chain amino acids—leucine,

isoleucine, and valine: cancer with

malnutrition, liver disease

Arginine and methionine: burn patients

Cysteine: recovery from protein-energy

malnutrition

Dangers of self-medication


Energy Source

Seldom used for energy in the fed state

May be used for energy in the fasting state

Nitrogen-containing amino group must be

removed

Remaining keto acid is converted to glucose

or fat


Protein and Nitrogen Balance

Concept of Balance

Interdependent checks and balances

Constant ebb and flow of tissue building and

breakdown

Coordinated activity to maintain normal

function


Body Protein Reserves

The average man contains approximately 11

kg of protein

Nearly half this protein (43%) is found in skeletal

muscle

The remainder is in skin, blood, kidney, liver,

brain, and other organs


Body Protein Reserves – Cont’d

Body distribution of protein changes with

growth and development


Body Protein Reserves – Cont’d

Labile protein reserves

Are easily broken down to meet immediate needs

Make up only about 1% of total body protein

Are intended to provide amino acids for an

emergency


Protein Balance

Finely tuned mechanisms control protein

balance and regulate protein synthesis and

breakdown across all body tissues


Protein Balance – Cont’d

Sometimes described as nitrogen balance

Net result of nitrogen gain and loss across all body

tissues


Protein and Nitrogen Balance

Protein balance

The steady state that exists between protein

synthesis (anabolism) and protein breakdown

(catabolism)

Protein turnover

Higher rate: intestinal mucosa, liver, pancreas,

kidney, plasma

Lower rate: muscle, brain, skin

Very slow: structural tissues such as collage

Higher in infants and children


Protein and Nitrogen Balance –

Cont’d

Protein compartments

Tissue protein

Plasma protein

Metabolic amino acid pool


Protein Quality

Evaluating Food Proteins

Protein quality depends on the following two

characteristics:

Protein digestibility

Amino acid composition


Protein Quality – Cont’d

Comparing Food Proteins

Nutritive value of a food protein = its amino

acid score

A value based on both its digestibility and amino

acid composition



Comparing Food Proteins – cont’d

When evaluating an amino acid score, it is

important to identify the limiting amino acid(s)

The limiting amino acid(s) is any indispensable

amino acid present in a lower amount than

recommended in the Amino Acid Reference

Pattern




The Amino Acid Reference Pattern indicates

the appropriate proportion of each

indispensable amino acid needed to produce

a body protein




Other methods used in animal studies to

evaluate protein digestibility and composition:

Biologic value (BV)

Net protein use (NPU)

Protein efficiency ratio (PER)


Amino Acid Content of Plant and

Animal Foods

Complete proteins: animal foods, which contain

all of the indispensable amino acids in the

amounts and ratio needed to support protein

synthesis

Include eggs, milk, cheese, meat, poultry, and fish


Amino Acid Content of Plant and

Animal Foods – Cont’d

Incomplete proteins: plant proteins

Supply less than the required amount of one or more

indispensable amino acids or are missing an

indispensable amino acid


Factors Influencing Protein

Requirements

Protein Quality

Protein requirements are influenced by the

quality of dietary protein and its amino acid

pattern

Protein Digestibility

Effects of food preparation

Digestibility and absorption is influenced by

the time interval between meals, with longer

intervals lowering the competition for

available enzymes and absorption sites



Requirements – Cont’d

Tissue Growth

Periods of rapid growth—as for fetal and

maternal tissues in pregnancy—require

added protein

Energy Content of the Diet

Sufficient amounts of carbohydrates must be

available to meet energy needs so dietary

protein can be used exclusively for tissue

building



Requirements – Cont’d

Health Status

Critical illness and diseases that increase the

rate of protein turnover and tissue breakdown

(catabolism) raise the protein requirement


Protein Requirements

Recommended Dietary Allowance (RDA) for

adults

Furnishes the protein and amino acids needed to

maintain or replace body tissues and replace

nitrogen lost via the urine, feces, and sweat, and

lost cells such as those lost from the inner surface

of the digestive tract


Protein Requirements – Cont’d

Recommended Dietary Allowance (RDA) for

adults – cont’d

0.8 g/kg body weight or 56 g/day for men

0.8 g/kg body weight or 46 g/day for women

An additional 25 g/day is needed for pregnancy

and lactation



Protein requirements of infants and children

vary according to age and growth patterns



Acceptable Macronutrient Distribution Range

10% to 35% of total kcalories


Protein Energy Malnutrition

Major health problem in many developing

countries where protein intakes are low in

both quantity and quality

More than 6 million children die each year from

protein-related deficiencies

Available protein sources in many developing

countries tend to be plant sources with poor amino

acid patterns


Protein Energy Malnutrition –

Cont’d

Marasmus

Chronic extreme starvation

Deficit of energy, protein, and micronutrients

Little or no body fat and extreme wasting

Kwashiorkor

Deficient in protein; low in energy and

micronutrients

Edema, hypoalbuminemia, skin lesions, fatty liver


Amounts and Types of Protein

Extremes

Research to increase indispensable amino acid

content of grains and vegetables

Excessive protein intakes in the United States


Low-Protein Diets

Children with protein-energy malnutrition

(PEM) have the following:

Poor growth with wasting (low weight for height)

and stunting (low height for age)

Impaired immune function

Chronic respiratory infections

Diarrhea


Low-Protein Diets – Cont’d

Low-protein intake during pregnancy

increases risk of a low-birth-weight infant

Clinical situations

Parkinson’s disease

Chronic kidney disease

Vegan diets


High-Protein Diets

Diets with 25% or more of total kcalories

(kcal) from protein appear to promote weight

loss, provide satiety, and can restore effective

energy regulation

Protein suppresses hunger to a greater

extent than carbohydrates

High protein intakes appear to protect muscle

mass


High-Protein Diets – Cont’d

Long-term effects on renal function:

Animal proteins increase blood flow to the kidney

Animal and vegetable proteins accelerate renal

impairment

Attend to fluid needs


Health Benefits of Plant Foods

Soy foods and legumes supply good-quality

protein and may help to prevent certain

chronic diseases

Soy foods are sources of phytochemicals that

may help prevent bone loss in older women


Health Benefits of Plant Foods –

Cont’d

Legumes lower risk of cardiovascular disease

Vegetable protein foods are lower in

saturated fat

Soybeans and chickpeas are good sources of

linoleic acid

Tree nuts and peanuts are rich in

antioxidants, trace minerals, essential fatty

acids, and phytochemicals


Nutritional Contributions of

Animal and Plant Proteins Animal sources

Provide indispensable amino acids

Meats provide iron and zinc

Dairy foods provide calcium, riboflavin, preformed

vitamin A

Vitamin B12 and vitamin D

Plant sources

Iron and zinc are less well absorbed

Add fiber and phytochemicals

Essential fatty acids

Fortified soy milk provides calcium and vitamin D


Vegetarian Diets

Reasons for following vegetarian diets

Religion

Sustainability of the food supply

Protection of the environment


Vegetarian Diets – Cont’d

Vegetarian diets are generally categorized as

follows:

Ovolactovegetarian: includes all plant foods, dairy,

and eggs

Lactovegetarian: includes all plant foods and dairy

Pescovegetarian: includes plant foods and fish

Vegan: includes plant foods only

Flexitarian: includes predominantly plant foods

with animal foods eaten occasionally



Nutritional Implications of Vegetarian Diets

Require careful planning, depending on type of

vegetarian diet

Phytates interfere with absorption of iron and zinc

Include vitamin C source to increase iron

absorption

Use of fortified foods

Effects of oxalate on calcium absorption



Planning vegetarian diets

Vegetarian Food Guide

Vegetarian diets and chronic disease

Lower coronary heart disease, obesity,

hypertension, and diabetes

Generally lower risk of cancer, uncertain effects

on specific cancers


Digestion-Absorption-Metabolism

The food proteins taken into the body must

be broken down into ready-to-use building

units—the amino acids

Mouth

The only digestive action on protein taking place in

the mouth is the mechanical effect of chewing



– Cont’d

Stomach

Three chemical agents in the gastric

secretions:

1. Pepsin

2. Hydrochloric acid

3. Rennin



– Cont’d

Small Intestine

Enzymes secreted by the pancreas and glands in the wall of the intestine help protein digestion: Pancreatic secretions: (1) trypsin, (2)

chymotrypsin, (3) carboxypeptidase

Intestinal secretions: (1) aminopeptidase, (2) dipeptidase


Absorption of Amino Acids

Characteristics of amino acid absorption

Requires active transport system

Enter portal blood


Absorption of Amino Acids –

Cont’d

Absorption of peptides and whole proteins

A few short-chain peptides and smaller intact

proteins escape digestion and are absorbed

Most undergo hydrolysis within the cells of the

intestinal mucosa and yield their amino acids

Protein molecules that pass into the blood intact

may play a part in the development of immunity,

protein sensitivity, and food allergies

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