Protein Flashcards
Elemental Composition of Proteins
C (Carbon)
H (Hydrogen)
O (Oxygen
N (Nitrogen)
Chemical Structure of Protein
Each protein molecule contains a number of amino acids
The amino acids are joined together by peptide links or bonds
A number of linked amino acids are called a polypeptide chain
Essential Amino Acids
Cannot be manufactured in the body. Must be obtained from food.
Examples of essential amino acids
Valine
Lysine
Leucine
Isoleucine
arginine
histidine
Children’s extra essential amino acids
arginine
histidine
Non-Essential amino acids
Glycine
Cysteine
Alanine
Asparagine
Protein Structure - Primary Structure
Primary structure is the sequence/order of the number of amino acids in a chain.
For example, insulin is made up of 51 amino acids arranged in a definite order.
Protein Structure - Secondary Structure
In a protein molecule, a polypeptide chain (or two different polypeptide chains) can be interlinked.
This creates a coiled or spiral shape.
These are known as cross-links or bridges and give proteins their properties - for example, the protein gluten is elastic.
Examples of Cross-Links
Disulphide links
This link occurs when two sulphurs join together.
The amino acid cysteine contains sulphur.
When two cysteine units, either in the same chain or two different polypeptide chains, are adjacent, a disulphide link may be formed.
Insulin has disulphide links.
Examples of Cross-Links
Hydrogen Bonds
Polypeptide chains can also be linked by hydrogen bonds.
The hydrogen in one chain joins with the oxygen in another chain.
Collagen has hydrogen bonds.
Define and tyrosine are capable of forming hydrogen bonds.
Protein Structure - Tertiary Structure
Tertiary structure relates to the pattern of folding of the polypeptide chains.
The coiled or spiral shape of the secondary structure may then be folded over to form a globule: a three dimensional organisation of the polypeptide chain held firmly by links.
Tertiary structures may be either fibrous or globular.
Fibrous
The polypeptide chain, composed of long, narrow strands of amino acids, is arranged in a straight, spiral or zigzag shape.
Example: Gluten and collagen, which are insoluble (not easily denatured) in water
Globular
The polypeptide chain is arranged in a globe shape.
Example: Ovalbumin (egg white) and myoglobin, which are soluble in water (easily denatured)
Classification of Protein
Simple Proteins
Animal:
Fibrous (eg: collagen in skin, myosin in muscle)
Globular (eg: albumin in egg, myoglobin in meat)
Plant:
Glutelins (insoluble in water, soluble in acids and alkalis eg: gluten in wheat)
Prolamins (insoluble in water, soluble in alcohol eg: gliadin in wheat)
Classification of Protein
Conjugated Proteins
these proteins consist of amino acids and a non-protein molecule:
Lipoproteins (lipid + protein) eg: lecithin in egg yolks
Phosphoprotein (phosphate + protein) eg: caseinogen in milk
Sources of Protein - Animal
Cheese
Chicken
Meat
Fish
Eggs
Milk
Sources of Protein - Plant
Soya beans
TVP good
Nuts
Lentils
Leas
Beans
Cereals
Cheese
casein
Meat connective tissue
collagen
Meat fibres
myosin/albumin/actin
Meat bones
gelatine
Fish
myosin/collagen/actin
Eggs
albumin
Milk
lactalbumin/caseinogen
Wheat
gluten
Properties of protein
Denaturation
Solubility
Maillard Reaction
Elasticity
Gel forming/Gelling
Foam Formation
The effects of dry and moist heat on protein
Coagulation
Colour Change
Maillard Reaction
Tenderising
Overcooking causes proteins to become indigestible
The effects of dry and moist heat on protein
Coagulation
Proteins set or harden when subjected to dry or moist heat.
Eg: Egg proteins solidify upon cooking (boiling/frying)
The effects of dry and moist heat on protein
Colour Change
Dry and moist heat make proteins change colour.
Eg: Myoglobin in meat turns brown (haematin)
The effects of dry and moist heat on protein
Maillard Reactions
Dry heat causes a browning of food when amino acids and carbohydrates react together
Eg: Roast potatoes
The effects of dry and moist heat on protein
Tenderising
Collagen in meat changes to gelatine, causing the fibres to fall apart and meat to become more digestible
Eg: lamb casserole
The effects of dry and moist heat on protein
Overcooking causes proteins to become indigestible
Prolonged cooking causes proteins to become indigestible.
Eg: overcooking meat will make it tough and difficult to digest
Biological Functions of Proteins
Structural Protein
Production of: Cell membrane, Muscle and skin (and growth and repair)
Biological Functions of Proteins
Physiologically active proteins
Production of:
Hormones
Enzymes
Antibodies
Blood proteins and nucleoproteins
Biological Functions of Proteins
Nutrient Proteins
Supply the body with essential amino acids
Excess protein can be used for energy
Deamination
Is using protein as a source of energy
Excess protein not required for growth and repair is deaminated in the liver
The NH2 group is removed, converted to ammonia and then urea, and is excreted by the kidneys.
The COOH group is oxidised to produce heat and energy.
Biological value of proteins
HBV
HBV proteins contain all essential amino acids
They’re known as complete proteins
HBV generally comes from animal sources but also from soya beans
Eg:
Eggs
Milk
Meat/Fish
Soya Beans
Biological value of proteins
LBV
LBV proteins contain only some of the essential amino acids
They’re known as incomplete proteins
LBV proteins generally come from plant sources but also from gelatine
Eg:
Rice
Wheat
Maize
Gelatine
Supplementary/Complementary value of protein
Eating two LBV protein foods together can ensure that all the essential amino acids are obtained.
This is very important in vegan diets, where no animal protein is eaten.
Beans on toast is an example of proteins supplementing or complementing each other.
Beans are high in lysine but low in methionine.
Bread (wheat) is low in lysine but high in methionine.
Energy value of protein
Protein should make up 15% of total energy value of the average diet
RDA is based on:
Body weight
Rate of growth
1g per kg of body weight
Protein digestion
Organ/Gland: Stomach
Secretion: Gastric juice
Enzyme: Rennin, Pepsin
Substrate: Caseinogen, Proteins
Proteins: Casein, Peptones
Protein digestion
Organ/Gland: Pancreas
Secretion: Pancreatic juice
Enzyme: Trypsin
Substrate: Peptones
Product: Peptides
Protein digestion
Organ/Gland: Lleum
Secretion: Intestinal juice
Enzyme: Peptidase
Substrate: Peptides
Product: Amino acids
Absorption and Utilisation of amino acids
Amino acids are absorbed through the villi of the small intestine into the bloodstream. The portal vein carries them to the liver.
In the liver:
They’re used to maintain and repair liver cells
They’re sent into the bloodstream to form new cells, repair damaged cells and manufacture hormones, enzymes, antibodies, blood proteins and nucleoproteins.
The excess is deaminated to produce heat and energy
Formation of peptide bonds
Peptide bonds form when a “H” from the NH2 group of one amino acid and the “OH” from the carboxyl group of another amino acid joins together to form a water molecule forming a peptide link in a condensation reaction
Hydrolysis
a reaction that involves the addition of water and enzyme action
Condensation reaction
a reaction that results in the loss of a water molecule
Draw the structure with a key
H
|
R——C——COOH
|
NH2
C- carbon
H- hydrogen
NH2 - amino group**
COOH - carboxyl group**
R - variable
Properties of protein - Denaturation
Heat causes coagulation. For example upon heating, the albumin in egg hardens/coagulates
Culinary application: protein coagulates when eggs are cooked
Acids lower the pH. For example, milk souring bacteria changes lactose to lactic acid which causes the caseinogen in milk to coagulate
Culinary application: Lactic acid bacteria is added to milk in cheese making
Enzymes denature protein. For example, rennin coagulates milk during cheese-making
Culinary application: Tenderising salts contain enzymes, which tenderise meats, proteolytic enzymes (papain) tenderise meat by converting collagen to gelatine
Mechanical action: Heat produced by friction slightly coagulates egg protein. For example, whisking egg whites causes them to foam.
Culinary application: Aeration of sponge cakes, soufflés and meringues
Properties of protein - Solubility
Most proteins are insoluble in water, except egg whites in cold water and collagen in hot water
Culinary application: Collagen is converted to gelatine in moist heat, which tenderises meat.
Properties of protein - Maillard Reaction
The maillard reaction is the browning of food caused by dry heat when amino acids and carbohydrates react together. It’d non-enzymes browning.
Culinary application: Browning of fried potatoes
amino acid + sugar + dry heat = attractive brown colour and flavour (eg:) roast potato
Properties of protein - Elasticity
Elasticity is a property of some proteins — for example gluten
Culinary application: gluten in wheat flour allows baked goods to rise
Properties of protein - Gell formation
Gelatine is a setting agent that is extracted from the bones of animals.
Gelatine can absorb large amounts of water forming a gel.
Culinary application: Gelatine is used as a setting agent in soufflés and mousses
Denaturation definition
Denaturation id the unfolding of the protein chain, resulting in an irreversible change in shape.
Coagulation of protein is an example of denaturation.
Denaturation is caused by:
Heat
Acids
Enzymes
Mechanical action
Causes of Denaturation: Heat
Heat causes coagulation. For example upon heating, the albumin in egg hardens/coagulates
Culinary application: protein coagulates when eggs are cooked
Causes of Denaturation: Acids
Acids lower the pH. For example, milk souring bacteria changes lactose to lactic acid which causes the caseinogen in milk to coagulate
Culinary application: Lactic acid bacteria is added to milk in cheese making
Causes of Denaturation: Enzymes
Enzymes denature protein. For example, rennin coagulates milk during cheese-making
Culinary application: Tenderising salts contain enzymes, which tenderise meats, proteolytic enzymes (papain) tenderise meat by converting collagen to gelatine
Causes of Denaturation: Mechanical Action
Mechanical action: Heat produced by friction slightly coagulates egg protein. For example, whisking egg whites causes them to foam.
Culinary application: Aeration of sponge cakes, soufflés and meringues
Properties of Protein - Foam Formation
When egg white is whisked, protein chains unfold and air bubbles form.
The protein chains entrap air, creating a foam.
Whisking also creates heat that begins to set the egg albumin.
This is known as a temporary foam.
It will collapse after a while unless heated to coagulate and set a permanent foam.
Culinary application: Meringues/Sponges
Biological Functions of Proteins - Deficiency of Structural Proteins
Delayed growth and healing
Biological Functions of Proteins - Deficiency of Physiologically active proteins
Body organs and systems malfunction
Easily infected
Biological Functions of Proteins - Deficiency in Nutrient Proteins
Lack of energy
Kwashiorkor Marasmus
