Protein Flashcards
Primary Structure of Protein
The primary structure refers to the sequence/order and the number of amino acids in a chain.
For example insulin has 51 amino acids arranged in a definite order.
Secondary Structure
In a protein molecule, a polypeptide chain (or two polypeptide chains) can be interlinked.
This creates a coiled or spiral shape. These links are known as cross-links or bridges and give protein it’s properties
For example the protein gluten is elastic.
An example of a Cross-Link would be a disulphide link.
This link occurs when two sulphur units 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.
Cross-Link Example
An example of a Cross-Link would be a disulphide link.
This link occurs when two sulphur units 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.
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)
Deamination
Deamination is using excess protein as a source of energy.
Excess protein not needed for growth and repair is deaminated in the liver.
The NH2 is removed, converted to ammonia then urea and is excreted by the kidneys as a waste product in urine.
The COOH group is oxidised to produce heat and energy.
Classification of protein - simple
Animal
Fibrous: collagen in skin, myosin muscle
Globular: albumin in egg, myoglobin in meat
Plant
Glutelins: insoluble in water, soluble in acids and alkalis (Eg:) glutenin in wheat
Prolamins: insoluble in water, soluble in alcohol (Eg:) gliadin in wheat, zein in maize
Classification of proteins - conjugated proteins
These proteins consist of amino acids and a non protein molecule.
Examples include:
Lipoproteins (Eg:) lecithin in eggs
Phosphoproteins (Eg:) caseinogen in milk
Lipid + protein
Phosphate + protein
Sources of protein
Animal
Cheese
Chicken
Meat
Fish
Eggs
Milk
Sources of protein
Plant
Soya beans
TVP food
Nuts
Lentils
Peas
Beans
Cereals
Cheese
Caesin
Meat connective tissue
Collagen
Meat fibres
Myosin/ actin/ albumin
Meat bones
Gelatin
Eggs
Albumin
Fish
Myosin/ collagen / actin
Milk
Lactalbumin
Caseinogen
Wheat
Gluten
Properties of protein
Denaturation
Solubility
Maillard reaction
Elasticity
Gel formation/forming
Foam formation
Denaturation
The unfolding of the protein chain, resulting in an irreversible change in shape
Coagulation is an example of denaturation
Denaturation - heat
Heat causes coagulation, upon heating the ablumin in egg gardens/coagulates.
Egg white coagulates at 60°C
Egg yolk coagulates at 68°C
Culinary application
Protein coagulates when eggs are cooked (Eg:) boiled, scrambled, fried
Moist heat (boiling) changes collagen to gelatine. This tenderises meat.
Denaturation - Acids
Acids lower the pH. For example milk souring bacteria change lactose to lactic acid which causes caseinogen in milk to coagulate
Culinary Application
Lactic acid bacteria is added to milk in cheese making
Denaturation - Enzymes
Enzymes denature protein. For example rennin coagulates milk during cheese making
Culinary Application
proteolytic enzymes (Eg:) Papin tenderise meat by converting collagen to gelatine
Denaturation - Mechanical Action
Heat produced by friction slightly coagulates egg protein. For example whisking egg white causes them to foam
Culinary Application
Aeration of sponge cakes, soufflés, meringues
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
Maillard Reaction
Maillard reaction is the browning of food caused by dry heat when amino acids and carbohydrates react together.
It is non-enzymic browning.
Culinary Application
Browning of fried potatoes, brown crusts on bread
Elasticity
Elasticity is a property of some proteins — for example gluten
Culinary Application
Gluten in wheat allows baked goods to rise
Gel formation/Gelling
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
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 whole, unless heated to coagulate and set as a permanent foam
Culinary Application
Meringues/ sponges
Biological functions of proteins
Structural
Production and growth and repair of:
Cell membranes
Muscle and skin
Deficiency: delayed healing/growth
Biological functions of protein
Physiologically active
Production of
Hormones
Enzymes
Antibodies
Blood proteins
Nucleoproteins
Deficiency: body organs and systems malfunction, easily infected
Biological functions of protein
Nutrient
Supply the body with essential amino acids
Excess protein can be used for energy
Deficiency: lack of energy, kwashiorkor marasmus
Biological value of proteins
HBV
HBV proteins contain all essential amino acids
They are also known as complete proteins
HBV proteins generally come from animal sources, but also from soya beans
HBV foods:
Eggs
Milk
Meat/fish
Soya beans
Biological value of proteins
LBV
LBV proteins contain only some of the essential amino acids
They are also known as incomplete proteins
LBV proteins generally come from plant sources but also from gelatine
LBV foods:
Rice
Wheat
Maize
Gelatine
Effects of dry and moist heat on protein
Coagulation
Colour change
Maillard reaction (fry)
Tenderising (moist)
Overcooking - indigestible
COMCT
Effects of dry and moist heat on protein
Coagulation
Proteins set or harden when subjected to dry or moist heat
Example: egg proteins solidify upon cooking (boiling, frying)
Effects of dry and moist heat on protein
Colour change
Dry and moist heat make proteins change colour
Example: myoglobin (red) in meat turns brown (haematin)
Effects of dry and moist heat on protein
Maillard reaction (dry heat)
Dry heat causes browning of food when amino acids and carbohydrates react together
Example: roast potatoes
Effects of dry and moist heat on protein
Tenderising (moist)
Collagen in meat changes to gelatine, causing the fibres to fall apart and the meat to become more digestible
Example: lamb casserole
Effects of dry and moist heat on protein
Overcooking - indigestible
Prolonged cooking causes proteins to become indigestible
Example: overcooking meat will make it tough and difficult to digest
Supplementary/complementary value of promete in
Eating two low biologung value protein foods together can ensure that all 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 is low in lysine but high in methionine
Stomach
Secretion: gastric juice
Enzyme: rennin, pepsin
Substrate: caseinogen proteins
Product: casein peptones
Pancreas
Secretion: pancreatic juice
Enzyme: trypsin
Substrate: peptones
Product: peptides
Ileum
Secretion: intestinal juice
Enzyme: peptidase
Substrate: peptides
Product: amino acids
Amino acids in the liver
They are used to maintain and repair liver cells
They are sent into the bloodstream to form new cells, repair damaged cells and manufacture hormones, enzymes, antibodies, blood proteins, nucleoproteins
The excess is deaminated to produce heat and energy
