Foodchem Lecture 10 Pics

• The aliphatic category amino acids (besides glycine and alanine)
• Valine, Leucine, Isoleucine
• Side chains are relatively unreactive and their major feature is their increasing hydrophobicity as the chain lengthens

• Serine
• Hydroxylic side chain (left side)
• OH groups can serve as a hydrogen bonding site - important structurally

• Threonine
• Hydroxylic side chain (bottom left)
• OH groups can serve as a hydrogen bonding site - important structurally

• OH groups can serve as a hydrogen bonding site - important structurally
• OH is also a generally reactive moiety and can be used to attach groups

• OH groups can serve as a hydrogen bonding site - important structurally
• OH is also a generally reactive moiety and can be used to attach groups
• In this case, it gives it the ability to form covalent diester linkages with phosphates
• Common in the case of milk proteins
• Can play an important role in the stabilizing the 3D structure of a protein

• Aspartic acid
• pKb = 3.87 (COOH/COO-)
• Carboxylic amino acid
• Free carboxyl group is available which is capable of ionizing, can provide the protein with an overall negative charge depending on the pH

• Glutamic acid
• pKy = 4.27 (COOH/COO-)
• Carboxylic amino acidFree carboxyl group is available which is capable of ionizing, can provide the protein with an overall negative charge depending on the pH
• Beta carboxyl group of glutamic acid is 50% ionized at pH 3.87 while the gammaa COOH of glutamic acid is 50% ionized at 4.27

COO- charges increasing as pH rises

• Asparagine
• Amide amino acid
• Side chain of the amides of aspartic/glutamic acid do not ionize to any extent and are relatively unreactive
• Can play an important role in non-enzymatic browning (Maillard) reactions

• Glutamine
• Amide amino acid
• Side chain of the amides of aspartic/glutamic acid do not ionize to any extent and are relatively unreactive
• Can play an important role in non-enzymatic browning (Maillard) reactions

• Asparagine --------> Aspartic acid
• These amide AA are readily hydrolyzed under conditions of heat and/or acidic conditions to their corresponding acids
• Thus proteins containing these AA are susceptible to processes involving heat and acidity

• Lysine
• Has 2 amino groups, with the epsilon (e) group being one of the most reactive groups present in many protein systems

• Arginine
• Has very basic guanido group which has a pKa of 12.5
• Proteins with significant amounts of lysine and arginine tend to have high isoelectric points

• Histidine
• Imidazole group
• pKi (Imidazole) = 6.0 (NH₂+/NH)
• Histidine "R" group is a weakly basic amino acid (50% ionized at pH 6)

• Phenylalanine
• One of the 3 aromatic amino acid
• All have an aromatic ring in their structure
• Hydrophobic
• Responsible for UV absorbance properties of most proteins at 280nm

• Tyrosine
• One of the 3 aromatic amino acid
• All have an aromatic ring in their structure
• Hydrophobic
• Responsible for UV absorbance properties of most proteins at 280nm
• Tyrosine also has 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

• Tryptophan
• One of the 3 aromatic amino acid
• All have an aromatic ring in their structure
• Hydrophobic
• Responsible for UV absorbance properties of most proteins at 280nm
• Tryptophan is considered to be somewhat basic, and its positive charge is suppressed only beyong pH 10 (NH₂+/NH)

• Cysteine --------> Cystine
• eine = 1
• Cysteine is very reactive and can undergo oxidation-reduction reactions to form the dimer cystine or be reduced to cysteine
• There are 3 sulfur amino acids, cysteine, its dimer cystine, and methionine

• Methionine
• The other sulfur amino acid
• Sulfur group is not capable of ionizing but is relatively unstable and can decompose when heated to release hydrogen sulfide (H₂S), a reaction that can also occur with cysteine and cystine

• Proline (NH₂+ at pH < 7; NH at pH > 7)
• Imino acid - not true amino acid because does not contain a true amino group (NH₂), but an imino (NH) group
• Disrupt natural tendency for proteins to form an alpha helix when they are incorporated into the protein chain

• Hydroxyproline (OH is reactive)
• (NH₂+ at pH < 7; NH at pH > 7)
• Imino acid - not true amino acid because does not contain a true amino group (NH₂), but an imino (NH) group
• Disrupt natural tendency for proteins to form an alpha helix when they are incorporated into the protein chain

Protein hydrolysates (acid or enzymatic hydrolysates) are used extensively as flavoring agents to provide a meaty or brothy flavor to soups, etc.)

• MSG is also associated with flavor development in some fruits and vegetables - like tomatoes
• This graph shows how glutamate levels in tomatoes rise during the ripening process to over 100mg per 100 mls of juice

• Formation of a peptide bond
• Special form of an amide bond where the alpha amino group of one AA is linked to the alpha carboxyl group of the next AA

• Trans C=O and NH
• If we link several amino acids together via a peptide bond, a specific structure develops
• The carboxyl oxygen and amino hydrogen of the peptide bond are always trans along the chain relative to each other

• If there was only one level of structure of proteins, they would all be long spaghetti-like molecules, however due to the natural bond angles, the singular L-amino acid configuration and steric hindrance of the 'R' groups
• The molecule is shortened and forms a spring-like alpha helix, the secondary level of structure

• Alpha helix secondary structure
• Due to the L-chiral form of AA, natural bond angles and steric hindrance - as the protein polymer elongates it tends to twist upon itself, producing a helical structure

• Alpha helix secondary structure
• Due to the L-chiral form of AA, natural bond angles and steric hindrance - as the protein polymer elongates it tends to twist upon itself, producing a helical structure
• Structure is stabilized by hydrogen bonding due to the repetitive proximity of the peptide carbonyl oxygen and the hydrogen of the peptide nitrogen

Structure is stabilized by hydrogen bonding due to the repetitive proximity of the peptide carbonyl oxygen and the hydrogen of the peptide nitrogen