Proteins (Food Sci & Nutrition Sci aspects) Flashcards
Identify and illustrate the basic amino acid structure, and explain the significance of the side chain.
Amino Acid Structure
- The side chain determines amino acid properties
(whether it is hydrophilic, hydrophobic or charged)
- Hydrophobic amino acids have carbon-rich side
chains, which don’t interact well with water - Hydrophilic or polar amino acids interact well with
water
> Their side chains normally contain polar groups
> Main interactions are hydrogen bonds formed with
water molecules - Charged amino acids have side chains with charged
species, they can interact with other oppositely
charged amino acids or other molecules
Identify, based on the chemical structure, hydrophilic, hydrophobic and charged amino acids.
Yes
State and describe the 4 levels of protein structural organization.
Protein structure:
1. Primary structure: linear sequence of amino acids as
encoded by DNA
> The amino acids are joined by peptide bonds, which
link an amino group and a carboxyl group
> A water molecule is released each time a bond is
formed
2. Secondary structure: The protein chains often fold
into two types of secondary structures; alpha
helices, or beta-sheets:
> Alpha helix: A right-handed coil stabilised by
hydrogen bonds between the amine and carboxyl
groups of nearby amino acids
> Beta sheet: hydrogen bonds stabilise two or more
adjacent strands
3. Tertiary structure: 3D shape of protein chain (SIDE CHAIN INTERACTIONS START OCCUR HERE)
> The shape is determined by the characteristics of
the amino acids making up the chain
> It is generally stabilised by outside polar hydrophilic
hydrogen and ionic bond interactions between
nonpolar amino acid side chains
> In some instances, disulphide bonds, covalent
linkages between the sulphur-containing side chains
of cysteines
> Role in biological environments:
- Charged amino acids allow proteins to interact
with molecules that have complementary charges
- Many proteins form globular shapes with
hydrophobic side chains sheltered inside, away
from the surrounding water
- Membrane-bound proteins have hydrophobic
residues clustered together on the outside so that
they can interact with the lipids in the membrane
- Functions of proteins rely on their 3D structure -
e.g. haemoglobin forms a pocket to hold heme, a
small molecule with an iron atom in the centre
that binds oxygen
- Quaternary structure:
> Two or more polypeptide chains can come together
to form one functional molecule with several
subunits
> E.g. the 4 subunits of haemoglobin cooperate so
that the complex can pick up more oxygen in the
lungs and release it in the body
Explain why the 3-dimensional shape of proteins are important for their functionality.
Yes
State the forces and interactions that stabilize the native structure of protein.
The native structure of a protein is the result of:
- Repulsive and attractive forces
- Interactions among the amino acid side groups of
proteins
- The environment the protein is in
Forces that stabilise protein structure
> Hydrophobic interactions
> Electrostatic interactions
- Van der Waals interactions
- Hydrogen bonds
- Ionic interactions
> Covalent bonds or chemical cross-links
- Disulphide bonds
State chemical and physical protein denaturants.
When the proteins are subjected to conditions that disturb their stability, they undergo denaturation
> Protein denaturation:
- Refers to changes in the secondary and tertiary
structure of the protein, while the primary remains
unchanged
- Protein denaturation under controlled conditions is
used to obtain various product textures and flavours
> A denatured protein will:
- Undergo physical and chemical changes
- Lose its biological activity
- May lose solubility
- Increase the tendency to aggregate
Protein denaturation is often beneficial in food processing:
o Often improves digestibility of proteins
o Deactivate antinutritional factors
o Inactivate deteriorative enzymes
o Serves as a method for monitoring food process
quality
o Improve functional properties of food proteins that
are used as ingredients in processed food
These are generally factors that contribute to the changes in the environment surrounding the protein which result in disruption of interactions that stabilise the secondary and tertiary structure:
> Physical Denaturants:
1. Temperature-induced
Denaturation temperature is unique to the protein
Thermal stability of proteins is determined by the
amino acid composition
Hydrophobic amino acid residues provide for
higher stability than proteins with more hydrophilic
amino acids
Some proteins also denature at cold temperatures,
cold-induced denaturation is much less studied.
The effect of cold temperatures on proteins is
often reversible, contrary to high temperatures.
Both high and low temperatures can cause proteins
to denature
2. Shear-induced
Exposure to mechanical shear forces generated by
forces such as shaking, whipping, mixing,
sonication, vortexing, flow through conduits,
centrifugation
3. High-pressure induced
> Chemical Denaturants
pH-induced
Organic solvent induced
Salts and other additives
Briefly explain the role of proteins in food emulsions.
Emulsifying properties
o Proteins in general act as macromolecular
emulsifiers in the formation and stabilisation of
many food dispersions
> Proteins help mix and stabilize ingredients that don’t usually blend well, like oil and water. They act as macromolecular emulsifiers, meaning they keep mixtures smooth and prevent them from separating.
Briefly explain how protein gels are formed.
Gel formation: Chemistry of protein gel formation
1. In a gel, the liquid prevents the 3D matrix from
collapsing into a compact mass and the matrix prevents the liquid from flowing away
2. Gels are formed when partially unfolded proteins
develop uncoiled polypeptide segments that interact
at specific points to create a 3D cross-linked network 3. Partial unfolding of proteins with slight changes in
secondary structure is required for gelation
4. Heating, treatment with acids, alkali can contribute to
partial unfolding of the native structure
5. The 3D protein network forms as a result of protein-
protein and protein-solvent (water) interactions
6. These interactions and gelation are accelerated at
high protein concentrations because of more intense
intermolecular contacts
7. A higher amount of cross-linking in protein gels
provides fluidity, elasticity, and flow behaviour of
gels, formation of the rubbery nature of protein gels
What is protein hydrolysis and why is it important in food processing.
- Condensation reaction forms the peptide bond by removing a water molecule
- Hydrolysis reaction adds a water molecule to break a peptide bond
- Different from protein denaturation where the primary structure was unchanged*
Protein hydrolysis:
- Use food-grade enzymes called proteases or peptidases
> Endoproteases hydrolyse the protein in the middle of the molecule, forming peptides of more or less the same size when a water molecule is added to a peptide bond
- Exopeptidases hydrolyse the peptide at the end, removing an amino acid
small peptides contribute to the basic aromas of raw materials, and free amino acids act as enhancers of taste and aroma
Differentiate between and list the essential and non-essential amino acids and explain the synthesis of non-essential amino acids. Also define conditionally essential amino acids.
Essential amino acids are those that the body cannot synthesise - must be consumed through food
Non-essential amino acids can be synthesised by body
- If a particular non-essential amino acid is not
readily available, cells can make it from another
amino acid or keto acid
- If an essential amino acid is missing, the body may
break proteins to obtain it
1) Using a nitrogen source to synthesise non-essential
amino acids
- Cells can make non-essential amino acids from a
keto acid and from a nitrogen source (Eg.
Ammonia)
2) Transamination to synthesise non-essential amino
acids
- The body can transfer amino groups (-NH2-) from
an amino acid to a keto acid, forming a new non-
essential amino acid and a new keto acid.
-Transamination requires the coenzyme vitamin
B6.
Essential= My tall handsome vegan friend (Ph) is tired of vegan leaves
Non-essential= ACG & proline, serine, tyrosine
Outline the stages of protein digestion and absorption
- Protein digestion in the stomach
- Partial breakdown (hydrolysis) of proteins begins in
the stomach - Hydrochloric acid denatures proteins so that
digestive enzymes can attack the peptide bonds - Hydrochloric acid activates the digestive enzyme,
pepsin, from its inactive form (pepsinogen) - Pepsin cleaves large polypeptides into smaller
polypeptides and some amino acids
> Chemical Digestion of Proteins - Gastric juice includes HCL acid and pepsin enzyme
> Mechanical digestion - contractions (peristalsis)
- Partial breakdown (hydrolysis) of proteins begins in
- Protein digestion in the small intestine
- Proteases are stored as inactive proenzymes. The
pancreas stores trypsin as trypsinogen and
chymotrypsin as chymotrypsinogen - Once chyme enters the small intestine,
enteropeptidases convert trypsinogen to the active
form of trypsin - Trypsin cleaves peptide bonds next to
the amino acids lysine and
arginine
- Trypsin then converts: - Chymotrypsinogen to chymotrypsin
> Cleaves peptide bonds next to
the amino acids phenylalanine,
tyrosine, tryptophan, methionine,
asparagine and histidine - Procarboxypeptidases to carboxypeptidase
> Cleave amino acids from the acid
(carboxyl) ends of polypeptides - Pancreatic proteases break down proteins to oligo-,
tri- and di-peptides - These are further broken down to single amino acids
by intestinal peptidases
- Proteases are stored as inactive proenzymes. The
- Elastase and collagenase
> Cleave polypeptides into smaller
polypeptides and tripeptides - Intestinal tripeptidases
> Cleave tripeptides to dipeptides
and amino acids - Intestinal dipeptidases
> Cleave dipeptides to amino acids - Intestinal aminopeptidases
> Cleave amino acids from the
amino ends of small polypeptides
(oligopeptides)
- Protein absorption
- Specific carriers in the membranes actively transport
amino acids (and some di- and tri-peptides) into the
intestinal cells - Intestinal cells may use the amino acids for energy
or to synthesise new proteins - Amino acids not needed by intestinal cells are
transported to the surrounding capillaries where
they head to the liver
- Specific carriers in the membranes actively transport
Describe the 9 major roles of proteins in the body (e.g. building materials, enzymes, hormones, regulator of fluid and electrolyte balance, acid-base regulation, transporters, antibodies)
- Structural purposes
- provides building blocks for most of the bodies
structures - e.g. muscles, blood vessels, skin, bone, hair and
replacement of dead cells
- provides building blocks for most of the bodies
- Enzymes facilitate chemical reactions
- proteins catalyse metabolic reactions through
anabolic and catabolic enzymes
- proteins catalyse metabolic reactions through
- Hormones
- chemical messenger molecules secreted by endocrine glands (pituitary) as well as some organs (pancreas)
Examples:
- Oxytocin & prolactin: support lactation
- Insulin & glucagon: regulate blood glucose
- Thyroxine: regulate body’s metabolic rate
- Calcitonin & parathyroid hormone: regulate blood calcium
- Antidiuretic hormone: regulate fluid and electrolyte balance
- Regulators of fluid balance
- proteins regulate the quantity of fluid in the blood,
cells and interstitial space - fluid flows freely between compartments but
proteins can’t - cells attract water as fluid follows the protein
gradient via osmosis
- proteins regulate the quantity of fluid in the blood,
- Antibodies
- molecules that combat antigens (foreign invaders)
- Acid-base regulators
- Proteins act as acid-base buffers, taking up or
releasing H+ ions from body fluids - Prevents acidosis and alkalosis which is life-
threatening
- Proteins act as acid-base buffers, taking up or
- Transport
- Pumps in cell membranes
- Carrier proteins in the blood to transport lipids, fat
soluble vitamins, oxygen and minerals - proteins attached to lipoprotein particles help
suspend the particle in blood (water) to travel
around body
- Source of glucose and energy
- Proteins sacrificed to provide glucose and energy (during starvation or insufficient carbohydrate intake)
- Blood clotting
- during blood vessel injury, collagen is exposed
- platelets adhere to exposed collagen
- substances are secreted to let more platelets
aggregate and initiate a cascade of reactions leading
to a blood clot
> Process forms fibrin strands (protein) that cross link
platelets and red blood cells to form a stable clot
> Fibrinogen and prothrombin are two protein
clotting factors found in blood
Describe what is meant by positive protein (nitrogen) balance, negative protein balance, and protein equilibrium and the proccesses that occur in postive protein balance.
Nitrogen balance and protein balance are essentially the same because protein is the primary source of nitrogen in the body, and measuring nitrogen intake and excretion provides a good indication of protein metabolism and status.
In a healthy adult, when protein degradation = protein synthesis, “Nitrogen balance”
- Positive nitrogen balance occurs when people take in more protein (Nirogen) than being excreted: Nin > Nout
o E,g,. childhood, recovery from illness, pregnancy
- Negative nitrogen balance when people lose more protein (nitrogen) than being consumed: Nin < Nout
o E.g. occurs in illness, starvation, and inadequate carbohydrate intake
Positive protein balance:
Deamination and elimination of amino acids
1) Using amino acids to make glycogen and fat
- There are no specialised storage sites for protein.
- When protein intake exceeds demand, protein is
converted to glucose or ketone bodies to be
stored as glycogen or fat. ~10-15% of dietary
protein is used as energy
2) Deaminating amino acids
- Amino acids are striped of their amino group,
resulting in ammonia and a keto acid. Keto acids
can be used for energy metabolism (TCA cycle)
fat production
3) Converting ammonia to urea
- Excess ammonia is toxic (upsets acid-base
balance). The liver combines ammonia with CO2
to make urea, a much less toxic compound.
4) Excreting urea
- Urea is excreted in solution (water)
Apply the three methods of assessing protein requirements (% energy, absolute intake in g/day and intake for bodily needs in g/day/kg body weight)
- Acceptable Macronutrient Distribution Range (% energy)
- Recommended Dietary Intake (RDI) (Absolute intake in g/day AND g/day/kg)
Identify food sources of protein, distinguish between high- and low- quality proteins and explain how plant-based diets can still meet the body’s nutritional needs.
- Protein is found in meat, milk, eggs, legumes and
many grains and vegetables
Describe how protein-energy malnutrition can lead to disease and the possible health consequences of too much dietary protein.
Explain the functional properties (affect texture) of food proteins and what impacts it.
Functional properties of food proteins are impacted by:
1. Intrinsic properties of protein
Amino acid composition
Protein structural organisation (primary,
secondary…)
Shape
Size
- Extrinsic factors coming from the food system
Food constituents
Food processing and external conditions
Functional Properties of Food Proteins:
o Food protein solubility
- Protein solubility in food through hydrophilic interactions
o Water-holding capacity of proteins
- ability to interact with water through hydrogen bonds
o Fat-absorption capacity
- falvour binding and fat retention through hydrophobic binding
o foaming properties
- protein adsorption at the interface coating air cells
> Emulsifying properties
- protein adsorption at interface coating oil droplets
o Gel forming properties
- gel formation through network formation
Name 4 groups of proteins plant proteins contain.
Most plant sources (mainly grains and seeds) including soy contain four groups of proteins:
o Storage proteins (50-90% total seed proteins)
o Enzymes and enzyme inhibitors (>1% of total protein) involved in metabolism
o Structural proteins including both ribosomal and chromosomal
o Membrane proteins
Explain the two factors which determine the protein quality of food.
Nutritional quality of food proteins:
- A high-quality diet supplies all essential amino acids in the right proportion
-The two factors which determine the protein quality of a food are:
1. How well the essential amino acid content match the requirement of the human body
2. How easily the protein is digested and absorbed into the body for utilisation
Describe the difference between “high-quality proteins” , limiting amino acids and complementary proteins.
- High-quality “complete” proteins contain all 9
essential amino acids. This includes:
> Most animal food: milk, cheese, eggs, poultry, fish
and meat
> A few plant foods: tofu and quinoa - Limiting amino acids are those essential amino acids
that are supplied in less than the amount needed to
support protein synthesis. The four most likely amino
acids to be limiting are: lysine, methionine, threonine,
tryptophan
> Individual plant sources of protein are considered
lower-quality (“incomplete proteins”) as they are
limited in one or more essential amino acids - Complementary proteins are foods that, when eaten
together, can supply the essential amino acids that
are missing in the other (e.g. beans and rice,
hummus on wholemeal pita, baked beans on toast)
The best guarantee of amino acid adequacy is to eat
foods containing high-quality proteins or mixtures of
complementary proteins
What are the 5 ways to measure protein quality?
- Amino acid scoring
- This is the ratio of a gram of the limiting amino acid
in a test food to the same amount of the
corresponding amino acid in a reference protein
(eg, whole-egg protein) multiplied by 100. Scoring is
based on the limiting amino acid (the one that falls
shortest compared to egg)
- This is the ratio of a gram of the limiting amino acid
- Biological value (BV)
- A measure of the proportion of retained protein
from food (that which gets incorporated into the
proteins of the body) to the amount of protein
that is absorbed. It summaries how readily the
broken-down protein can be used in protein
synthesis in the cells of the organism.
- A measure of the proportion of retained protein
- Net protein utilisation (NPU)
- The ratio of amino acid converted to proteins
(retained) to that which is supplied (consumed) 4.
- The ratio of amino acid converted to proteins
- Protein efficiency ratio (PER)
- Based on the weight gain of a test subject divided
by its intake of a particular food protein during the
test period
- Based on the weight gain of a test subject divided
- Protein Digestibility Corrected Amino Acids Score (PDCAAS)
- A method of evaluating the protein quality based on
both the amino acid requirements of humans and
their ability to digest them. The amino acid score is
multiplied by the food’s protein digestibility
percentage to determine PDCAAS.
state difference between peptide, protein and polypeptide
peptide = short amino linear sequence
polypeptide = linear amino sequence without 3D conformation
protein = complete biological molecule stable conformation
Compare fibrous and globular proteins and give examples.
Proteins are either fibrous and structural or globular and functional
- Fibrous
- long and narrow
- repetitive amino acid sequence
- less sensitive to pH and temperature
- generally insoluble in water
- e.g. myosin, actin, keratin, elastin, collagen, fibrin - Globular
- round and spherical
- irregular amino acid sequence
- more sensitive to pH and temp changes
- generally soluble in water
- e.g. enzymes, immunoglobulins, haemoglobin, insulin, cell messengers
Outline protein synthesis and the impact of sequencing errors.
Protein synthesis:
- making a protein is called gene expression
- synthesis of protein determined by DNA
- mRNA copied from DNA helps provide info on which complementary amino acids should bind
- tRNA lines up amino acids to mRNA
- The ribosomes move along the mRNA read the code and join amino acids together in the order dictated by the mRNA code to make a specific protein molecule
Sequencing errors
- cause alternate proteins to be made
- E.g in sickle cell anameia, valine replaces glutamic acid
resulting in haemoglobin shape to be altered iterfing
with ability to carry oxygen
