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Are the side chains of amino acids reactive?
No, they are relatively unreactive
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What is the major feature of the side chains of amino acids?
Increasing hydrophobicity as the chain lengthens
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What kinds of proteins will have a strong hydrophobic character?
Proteins with a lot of aliphatic amino acids
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What amino acids have hydroxylic side chains?
Serine and Threonine
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What do the OH groups in hydroxylic amino acids serve as?
Serve as hydrogen bonding site
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What is an example of how OH groups in hydroxylic amino acids is used?
To form covalent di-ester linkages with phosphates
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What is common in the case of milk proteins?
Ability of the OH group to form covalent di-ester linkages with phosphates
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What do covalent di-ester linkages with phosphates in milk proteins do?
Stabilize the three dimensional structure of a protein
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How are caseins unique?
They are very stable to heat
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Why are caseins very stable to heat?
Phophate esters
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In proteins, how are carbohydrates often attached to the OH group in serine or threonine?
By an O-glycosidic linkage
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What differentiates carboxylic amino acids?
A free carboxyl group is available
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Why is the carboxyl group of carboxylic amino acids important?
It is capable of ionizing, and can provide the protein with an overall negative charge depending on the pH
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The beta carboxyl group of glutamic acid is 50% ionized at what pH?
3.87
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The gamma carboxyl group is 50% ionized at what pH?
4.27
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How do dicarboxylic acid groups play an important role in protein structure?
They are very reactive and are important via charge associations
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What would the charge of most AA carboxyl groups be at pH 7 and above?
Most of these AA free carbozyl groups woul dbe egatively charged
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Which proteins tend to hae low isoelectric points?
proteins which contain significant amounts of aspartic and glutamic acid
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What is the sodium salf of glutamic acid?
MSG (mono sodium glutamate)
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What is MSG known for?
It is renowned for its flavor enhancing or potentiating proterties
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What is important about the side chains of amides of aspartic/glutamic acid?
The side chain of the amide of aspartic'glutamic acid do not ionize to any extend and are relatively unreactive
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What important reaction are the amide of aspartic and glutamic acid important for?
They can play an important role in non-enzymatic browning (maillard) reactions as the amide can serve as an attack point for reducing sugars in the maillard reaction
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What is generally considered to be reserve or inactive forms of aspartic and glutamic acid?
Amide form
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What happens to the amide of aspartic and glutamic acid in heat?
These amide AA are readily hydrolyzed under conditions of heat and/or acidic conditions to their corresponding acids
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In proteins, how are carbohydrates often attached to the amide group of asparagine?
By an n-glycosidic linkage
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What are the basic amino acids?
Lysine, arginine, and histidine
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How many amino groups does lysine have?
2
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What is important about the epsilon amino group of lysine?
it is one of the most reactive groups present in many protein systems
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Is epsilon-NH2 of lysine easiy affected by pH?
No, it is normally charged if the pH is neutral and is still 50% ionized at pH 10.8- thus requires a ery high pH to suppress the charge
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How does epsilon-NH2 of lysine react with reducing sugars?
Undergoes rapid reaction with reducing sugars (maillard reaction), which reduces the availability of this essential amino acid
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What very basic group does arginine contain?
Has a very basic guanido group wich has a pKa of 12.5
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Which proteins tend to have high isoelectric points?
Proteins with significant amounts of lysine and arginine tend to have high isoelectric points
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Which group does histidine contain?
Imidazole group
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His the histidine "R" group basic or acidic?
Histidine "R" group is a weakly basic amino acid (50% ionized at pH6)
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What are the aromatic amino acids?
Phenylalanine, tyrosine, and tryptophan
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What differentites aromatic amino acids?
All hae an aromatic ring in their structure
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What are aromatic amino acids responsible for?
these amino acids are responsible for the UV absorbance properties of most proteins at 280nm
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Are aromitic AA hydrophobic or hydrophilic?
Hydrophobic
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What important role to aromatic AA have?
Play an important role in hydrophobic associations and the three dimensional structure of protein
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Is tryptophan considered acidic or basic and positive, negative or neutral?
tryptophan is considered to be somewhat basice and its positive charge is suppressed only beyone pH 10
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What is important about tyrosine
contains an OH group which is reactive and capable of forming ester linkages with phosphate and is often used to conjugate proteins with sugars and polysaccharides
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What are the sulfur amino acids?
cysteine, its dimer cystine and methionine
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How is cysteine converted to cystine?
Cysteine is very reactive and can undergo oxidation-reduction reactions to form the dimer cystine or be reduced to cysteine
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What happens if a dimer forms between two cysteine in two separate proteins strands or within a single strand?
It forms a disulphide crosslink (R-S-S-R)
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What is cystine important for?
Plays an important role in the formation of covalent disulphide bonds in proteins and readily underghoes oxidation/reduction reactions dpeending on the environmental conditions
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Is the sulfur group of methionine stable?
No, it is not capable of ionizing but it is relatively unstable
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What happens when sulfur amino acids are heated?
When they are heated they can decompose to release hydrogen sulfide (H2S)
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What are the imino acids?
By definition proline an dhydroxy proline are not true amino acids becuase they do not contain a true amino group, but an imino group
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Why are imino acids unique?
When they are incorporated into the protein chain, they interrupt the natural tendency for proteins to form an alpha helix
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How do imino acids interrupt the natural tendency for proteins to form an alpha helix?
They cannot form intra-molecular hydrogen bonds
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Where are the AAs proline/hydroxyproline found extensively?
The AAs proline/hydroxyproline are found extensively in structural proteins such as collagen
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What are essential amino acids?
Amino acids that cannot be synthesized at all or fast enough for bodily requirements
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How does one overcome the problem of plant proteins being deicient in one or more of the essential amino acids?
Judicious blending of plant proteins can be used to overcome this problem or through amino acid supplementation (ie the addition of lysine to bread)
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What is non-protein nitrogen?
Free amino acids and peptides present in most food systems
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What has MSG been linked to?
Chinese Restaurant Syndrome
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What are protein hydrolysates used for?
Used extensively as flavoring agents to provide a meaty of brothy flavor to soups
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What is an example of a protein hydrolysate?
Soy sauve
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How is soy sauce produce>
Fermentation of soy beans or by acid hydrolysis
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How to sulfur amino acids provide flavor?
Provide flavor due to their breakdown when heated, mainly due to the release of hydrogen sulfide
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How are peptides responsible for strong bitter flavors?
Can arise in proteinaceous systems (cheese) due to the presence of low moleucular weight hydrophobic peptides
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What is generally considered to be the precursors of putrefactive odors and flavors in meat and fish?
Amino acids
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How are putrefactive compounds produced in meat and fish?
Such compounds are produced by the enzymatic deamination or decarbozylation of free amino acids by microbial enzymes
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What is the fundamental building blocks from which proteins are built?
Amino acids
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How are L-amino acids linked together?
Peptide bond
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What is a peptide bond?
Peptide bond is a special form od an amide bond where the alpha amino group of one AA is linked to the alpha carboxyl group of the next AA
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What is a conventional amide bond?
One formed between any OOH and NH2 group, other than those on an alpha carbon
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What conformation are the carboxyl oxygen and amino hydrogen of the peptide bond in primary structure
The carboxyl oxygen and amino hydrogen of the peptide bond are always trans along the chain relative to each other
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How is the alpha helix formed?
Due to the L-chiral form of AA, natural bond angles and steric hindrance- as the protein polymer elongates it tends to twist upom itsellf, producing a helical structure
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How is the alpha helix stabilized?
This structure is further stabilized by hydrogen bonding due to the repetitive proximity of the peptide carbonyl oxyen and the hydrogen of the peptide nitrogen
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What determines wether a helical strucal region forms?
Whether or not a helical structural region forms is a function of the amino acid compositio (sequence) within the protein
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Which proteins contribute to helix formation?
Most amino acids contriute to helix conformation, with the exception of proline and hydroxyproline (imino acids)- this is because they do not have a true amino group to contribute
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What are the two types of beta pleated sheets?
Parallel, Anti-parallel
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What types of proteins are beta pleated sheet super-structures generally associated with?
Fibrous proteins, such as beta keratin
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What are the three types of secondary protein structure?
Alpha helix, beta pleated sheert, suprahelix
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Where is the suprahelix found?
Found in collagen (very tough, inelastic protein)
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What is the general composition of the suprahelix?
Composed predominantly of glycine (33%), proline and hydroxyproline (25%)
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What is the basic unit of the suprahelix?
The basic unit is tropocollagen- the suprahelix is held together by hydrogen bonding between the chains, with the hydrogen bonds being supplied by glycine
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What supplies the hydrogen bonds in a suprahelix?
Hydrogen bonds are supplied by glycine
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What s disrupted collagen used for?
Disrupted collagen is extensively used as a gelling and binding agend in food systems
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How does collagen form a gel?
It forms a gel mainly by hydrogen bonding to form a 3 dimensional matrix to entrap water
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