P2.1 Reactions + denaturation Flashcards
are proteins more sensitive to their environment than lipids and carbs? + explanation
yes! can undergo drastic changes in structure (denaturation) under relatively mild conditions –> unfolding of tertiary structure and unraveling of secondary structure
is denaturation good or bad for protein?
can be good (ie eggs) and bad (overheat and precipitation) depending on one’s objective
definition of denaturation?
change in structure or conformation of a protein that does not involve breaking of peptide bonds (primary structure remains intact)
secondary and tertiary structure involves a balance of a number of ____________ bonds/interactions (4 types?) that hold protein in a space
- most interactions are ______ but their high numbers provide _________
- denaturation results from the __________ of these interactions
- non-covalent
- hydrophobic interactions, electrostatic interactions, hydrogen bonds, van der waals forces
- weak, provide stability
- disruption
sensitivity of proteins to denaturation is a function of many variables: (5)
- all also govern what of protein?
- amino acid composition
- chain length
- complexity of its structure
- prosthetic groups
- environmental conditions (ie pH)
all govern functionality of protein
casein has little _________ structure –> consequence? vs egg albumin
- little secondary structure
- does not denature readily even when boiled, vs globular egg albumin is very sensitive to temperature
when the conformation of a protein is changed, many of its ________ properties are also changed
physical
5 common changes observed when denaturation takes place
1) unfolding of the tertiary structure/unraveling of the secondary structure –> makes the peptide bonds more accessible to proteolytic enzymes
2) denaturation of protein generally results in a reduction in its solubility (bc hydrophobic core is out) –> can precipitate and form a gel
3) enzymatic activity may be decreased or lost
4) viscosity generally increases, gelation may occur
5) crystallization is no longer possible (no 2°, 3° or 4° structure)
if denaturation is extensive, what happens?
(3)
- globular form can be completely unraveled and its secondary structure badly disrupted
- many reactive groups like COOH, NH2, SH, etc. are exposed –> capable of reacting with each other, so that polymerization or aggregation takes place
- some proteins form gels when denatured, while others precipitate out of solution
difference between aggregation and polymerization?
- aggregation: protein coprecipitate with each other –> hydrophobic and sulfite bond
- polymerization: carboxyl and amino group attach from 2 different proteins –> need high temp and catalyst
most common factor contributing to denaturation? explanation
heat!
- increase in kinetic energy of molecules result in disruption of H-bonds, which are relatively weak
other important factors that contribute to denaturation (besides heat and more general) (4 + 1)
- changes in pH
- changes in ionic strength
- chemical agents (ethanol, acetone, urea)
- surface forces (ie. add air to proteins)
- combinations of above
can the disruption of 1 alpha helix change 3° structure?
yes
most proteins tend to be stable over a relatively large/narrow pH range
narrow
shift in pH affects ________ ________ of molecule, which affects the __________ bond contributions to the tertiary structure
- overall charge
- electrostatic bond
proteins with significant quantities of (3 aa) are especially susceptible to changes in pH
- aspartic acid
- glutamic acid
- lysine
many soluble proteins _________ when they are subjected to environmental conditions close to their _________ point
- why?
- precipitate
- isoelectric
- proteins fall out of solution because they don’t interact with water anymore –> positive and negative charges are balanced
contributors to denaturation (5)
1) effect of pH
2) Hydrogen bond breakers
3) Detergents
4) Organic solvents
5) surface forces
compounds that can effectively compete with _______ linkage for H-bonds can readily disrupt both ______ and _______ structure
- ex.: ?
- peptide linkage for H-bond
- both 2° and 3° structure
- ex.: urea
urea is commonly used to _______ proteins for analysis by _________, which is based on the fact that migration of proteins in an _______ _______ depends on their ____ and ___ ________ –> if not _________, their shape would be a confounding factor affecting their migration in the _______ ________
- denature
- by electrophoresis
- electric field
- depends on their size and net charge
- not denatured
- the electric field
what are good H-bond disruptors? how? mostly used for what?
alcohol and acetone
- by partial dehydration of a protein
- however, the degree of structure modification is not as severe as urea
- mostly used to precipitate enzymes from solution for isolation with minimal denaturation
detergents have both (2) moieties
- can also be called a ?
- ex? used in what?
- hydrophobic and hydrophilic moieties
- a surfactant
- ex.: sodium dodecyl sulfate (SDS) –> used in electrophoresis
detergents are able to bridge the (2) regions of protein = ________ of the structure of the protein
- hydrophobic and hydrophilic
- opening
organic solvent can make the environment hydrophobic/hydrophilic –> what does that do to the protein? (2)
- ex of organic solvent?
- hydrophobic
- can turn the protein inside out
- can also make enzymes work in totally opposite ways –> make lipase add FFAs to glycerol
- hexane
many proteins themselves are able to reduce _______ ______ –> results in the formation of a ______
- surface tension
- foam
in foam, what is trapped within what?
air is trapped within a protein/water matrix
- each air bubble has a thick/thin membrane (interface) that separates which 2 environment?
- at this interface, what do the proteins do?
- example
- thin membrane –> separates hydrophilic water and hydrophobic air
- proteins actively rearrange themselves structurally to reduce their free energy
- ie: with hydrophobic groups facing air + hydrophilic groups facing water –> can cause denaturation
denaturation can also take place at the ________ of a solution
surface
each protein is unique and its behavior depends on (2)
- its purity
- the presence of other constituents
key point is that proteins are very _______, readily changing their __________ as a function of the __________ conditions
- sensitive
- conformation
- environmental
although changes caused by denaturation are complex, they can be relatively ________
describe examples:
1) cooking egg
2) meringues
3) milk + acid
4) heating collagen
- consistent
1) boiling/frying an egg
2) meringues - egg white/sugar beaten to produce a denatured foam
3) milk+ acid produces a curd
4) heating collagen produces a gel
foods that contain appreciable amounts of protein generally undergo _______ when they are heated under ___-_________ conditions
- this is called the _________ reaction or ?
- browning
- low-moisture
- Maillard reaction or non-enzymatic browning
What is the Maillard reaction?
reaction of an aldehyde group of a reducing sugar and a free amino group of an amino acid/of a peptide/protein (ie Lysine)
is the Maillard reaction when avocado/bananas turn brown?
nope! that’s an enzyme that forms the brown/black
reducing sugar + amino compound (ie lysine) –> (name + structure)
- N glycosylamine
- reducing sugar –> top has an added NHR + double bonded O becomes OH
after initial Maillard reaction (that forms _________), the sugar portion undergoes a complex series of reactions called _________ rearrangement
- glycosylamine
- Amadori
what is the Amadori rearrangement? (2)
- involves loss of water from within sugar portion of the molecule
- then, sugar portion undergoes scission and breaks up into low-molecular weight compounds (producing flavor/aroma) that readily polymerize into brown pigments termed melanoidins
the brown pigments produced from Amadori rearrangement are called?
melanoidins
in terms of the protein, the net result of the Maillard reaction is that lysine will be ________ –> what does it mean?
bound
- amino group is not available because a sugar or a fragment of sugar may be attached
Bound lysine causes _______ _________ that limits ________ ________ of the protein
steric hindrance
- limits enzymatic digestion
what does limiting enzymatic digestion of a protein mean?
enzymatic action will stop 1 aa on either side of bound lysine, leaving an indigestible tripeptide, which is poorly absorbed and used by the body
AAAAAAALAAAAAAA
- enzyme can’t cut bond btw AL and LA –> leaving ALA tripeptide
why can non-enzymatic browning adversely affect nutritional value of proteins (also called reduction of ?)
- because of loss of lysine –> can’t be digested because stuck between a tripeptide
- reduction of protein efficiency ratio (PER)
what is protein efficiency ratio?
PER = weight gained/weight of protein consumed (measured in animal feeding trials)
explain how toasted cereals has a decreased nutritional value. solution?
- high heat + lots of sugar + lots of protein –> browning effect –> forms tripeptide that is indigestible = loss of lysine
- advertisement of eating toasted cereal with milk because milk is a very good source of lysine
which amino acids (except lysine) can also undergo reactions with reducing sugars at high temps? (Maillard reaction) (5)
- arginine
- tryptophan
- histidine
- glutamine
- asparagine
all have secondary amino group
if reducing sugars are mainly absent, what can occur at high temp? by formation of what?
- crosslinking of proteins
- formation of amide bonds btw COOH and NH2 groups on side chains of aa residues
cross-linking reactions –> peptide or amide bond that is formed? can it be hydrolyzed by digestive proteolytic enzymes?
amide! because it does not involve the alpha-carbon
- cannot be hydrolyzed because not a peptide bond
consequence of crosslinking of proteins?
enzymatic action will stop one aa on either side of cross-linked aa, leaving an indigestible hexapeptide
preparation of protein concentrates and isolates often calls for treatment of proteins with (2) to modify their properties
- what can be formed?
- alkali
- in presence of heat
- lysinoalanine
the unnatural aa __________ –> 2 characteristics, having shown to cause _______ failure in rats
lysinoalanine
- poorly absorbed + may be toxic
- renal failure
is the formation of lysinoalanine to be considered a major consequence?
no, unless protein is treated very harshly
strongly alkaline environments can lead to____________ of aa –> converting what to what? –> consequence?
- racemization
- converting L-amino acids to D-L mixtures
- lose effectivement of aa as building block for protein syntesis
2 degradative reactions of proteins?
- proteolysis
- putrefaction
consequence of proteolysis?
- reduction in molecular weight
- loss in functionality
what is proteolysis?
attack of proteins by proteolytic enzymes (abundant in living tissue and are secreted by microorganisms)
is proteolysis desirable?
- can be: tenderization of meat by action of papain (a proteolytic enzyme)
- also produces aroma in cheese (from small bitter hydrophobic peptides)
putrefaction is associated with ?
microbial spoilage of high-protein foods (meat/fish)
putrefaction involves what?
degradation of free amino acids, produced by proteolytic enzymes secreted by spoilage microorganisms
putrefaction: free amino acids are attacked by microbial (2 enzymes)
- deaminases
- decarbosylases
what do deaminases and decarboxylases do?
- deaminase: remove amine groups from free aa
- decarboxylase: remove carboxyl groups from free aa
products produced from deamination and decarboxylation are (3)
- volatile
- rather repulsive odoriferous compounds
- with reduced molecular weight
decarboxylase and deaminase most active at high or low pH?
decarboxylase: most active at low pH
deaminase: most active at high pH
- lysine treated with lysine decarboxylase produces ?
- ornithine treated with ornithine decarboxylase produces?
- both enzymes can be inhibited by ?
- lysine –> cadaverine
- ornithine –> putrescine
- by DFMO (difluoromethylornithine)
what is responsible for high protein systems becoming putrefactive –> becoming spoiled or inedible?
microorganisms are responsible because they have a good supply of deaminase and decarboxylase enzymes
_____ are especially susceptible of become putrefactive-like compounds because ?
fish
- they also undergo enzymatic reactions like conversion of trimethylamine oxide (TMAO = no smell) to trimethylamine (fishy smell) by trimethylamine-N-oxide reductase (when the fish dies)
*trimethylamine smells really bad –> protects you –> don’t eat it!
is trimethylamine oxide an aa?
no but the end product (trimethylamine) is putrefactive from a sensory standpoint
production of trimethylamine correlates very well with (2)
- rise in microbial population
- overall rise in putrefactive compounds
by measuring development of TMA in fish tissue, we have an indication of the _______ of the fish and potentially its ______-_______
- freshness
- shelf-life
most proteins are hygroscopic –> what does it mean?
have ability to bind water
what capacity of a protein is a very important functional property, especially from a textural standpoint?
2 examples
water-binding capacity
- milk proteins absorb moisture + retain it in bread = soft texture
- addition of soy protein to hamburger helps retain moisture, making burger juicier
water binding capacity (WBC) is strongly associated with (2)
- may also be associated with presence of a _________ as a prosthetic group
- ionic species present (NH3+ and COO-)
- hydrogen bonding sites (C=O, N-H) that are readily hydrated
- polysaccharide
water, having a strong _______ ________, is capable of forming a __________ _____ around polar groups
- dipole moment
- hydration shell
what is a moisture sorption isotherm?
- unit?
- illustrates the equilibrium moisture associated with a material as a function of relative humidity at a constant temperature
- unit: grams of H2O/100g protein
what do the 3 regions of the isotherm correspond to? + give characteristics
A) bound water: H-bonded to protein or a hydrate of an ionic species –> really an integral part of the molecular structure (monolayer) that in essence cannot be removed
B) water associated with bound water but more mobile –> still somewhat structured bc bound to bound water
C) “free water” –> unstructured, quite mobile and can be removed relatively easily –> region where proteins show their water absorption capacity
protein’s ability to bind water is a function of (3)
- its amino acid composition
- its overall structure (including prothetic groups)
- its charge
processing operations may involve substantial changes in factors that clearly affect protein structure –> 3 factors
- temperature
- pH
- ionic strength
2 processing operations to remove water
freezing and drying
freezing has very big/little effect on nutritional quality of proteins –> why?
- however, freezing can cause what?
- little effect bc no heat involved
- can cause major structural changes in proteins
in tissue systems (ie meat and fish), freezing can cause extensive _________ due to changes in (2) caused by (2)
- denaturation
- changes in ionic strength and pH
- caused by local concentration effects and the removal of water from microenvironment of protein molecule
freezing can cause extensive denaturation –> extent of denaturation is often dependent on what?
rate of freezing
in case of meat and fish, changes in ionic strength and pH are exemplified by gain/loss of water-binding capacity (manifested by ?) and often result in loss of desirable ________
- loss of WBC
- manifested by development of freezer drip (crystals btw meat and wrapper)
- desirable texture
deteriorative effects of dehydration processes may be due to (2)
denaturation and/or browning
what is one of the best methods of drying in terms of maintaining protein functionality, nutritional quality and enzyme activity?
- but ?
- freeze drying
- but very expensive
potential for denaturation and browning during freezing?
- some potential for denaturation
- little browning due to low temp of sublimation
in case of meat, ______ ______ is optimal in terms of retaining _______ _______ _______ of proteins
- freeze drying
- water binding capacity
what type of drying is used extensively for protein solutions?
- examples?
- denaturation?
- browning?
- spray drying
- milk and egg white
- some denaturation may take place due to higher temps and to shear at the nozzle
- some browning for egg white and milk bc of presence of glucose and lactose (Maillard reaction)
what is drum drying? causes denaturation and browning?
- fairly harsh treatment with extensive heating
- extensive denaturation and extensive browning if sugars are present
