42 Questions
What is the reason why cells are able to synthesize L-isomers of amino acids specifically?
The active sites of enzymes are asymmetric
What is the pH at which the overall charge on an amino acid molecule is zero?
Isoelectric point
What is the term for a molecule that can act as either an acid or a base?
Ampholyte
What is the term for a molecule that has both positive and negative charges, but is neutral overall?
Zwitterion
What is the term for a simple monoamino monocarboxylic α-amino acid, such as alanine, when fully protonated?
Diprotic acid
What happens to an amino acid at a pH more acidic than its isoelectric point?
It forms a positive ion
What is a characteristic of amino acids that enables them to resist changes in pH?
Buffering properties
What determines the charge of an amino acid at a given pH?
Its isoelectric point
What determines the role of an amino acid?
The structure of the R group
What is the main difference between amino acids in dietary proteins and structural proteins?
Their position (D/L) and attachment to alpha carbon
What is the main characteristic of nonpolar amino acids?
They are hydrophobic
What is the function of the peptide bond in proteins?
To connect amino acids
At physiological pH, what happens to the carboxyl group of amino acids?
It dissociates to form a negatively charged ion
How are amino acids classified based on their R groups?
By their polarity
What is the effect of the hydrophobic effect on protein structure?
It stabilizes protein structure
What is the simplest amino acid in terms of structure?
Glycine
What is the significance of color reactions in amino acids?
To recognize amino acids and thus proteins
What is the result of condensation between two amino acid molecules?
A dipeptide
What is the term for a chain of many amino acids?
Polypeptide
What is the linkage between amino acids in a peptide bond?
Substituted amide linkage
What is the direction of the amino acid chain in the figure?
From N-terminal to C-terminal
What is the role of ninhydrin in amino acid analysis?
To give a color reaction specific to all amino acids
What is the term for a chain of a few amino acids?
Oligopeptide
What is the orientation of the R groups in the peptide bond figure?
Outwards
What is the significance of the peptide bond in protein structure?
It joins amino acids together
What is the term for the end of an amino acid chain with a free amino group?
N-terminal
What is the role of disulfide bonds in protein structures?
Forming covalent links between parts of a polypeptide molecule
Which amino acid has a positively charged guanidinium group?
Arginine
What is the role of histidine residues in enzyme-catalyzed reactions?
Serving as proton donors/acceptors
Which of the following amino acids has a net negative charge at pH 7.0?
Aspartate
What is the function of γ-carboxyglutamate in proteins?
Binding to Ca2+
Which amino acid is a derivative of four lysine residues?
Desmosine
What is unique about selenocysteine and pyrrolysine?
They are added during normal protein synthesis through a highly specialized expansion of the standard genetic code
What is the role of basic amino acids in proteins?
They are proton acceptors
Which amino acid is essential in children but not in adults?
Histidine
What is the common feature of glycogenic amino acids?
They can be converted to α-ketoglutarate, pyruvate, oxaloacetate, fumarate or succinyl CoA
Which amino acid is converted to tyrosine?
Phenylalanine
What is the number of glycogenic amino acids?
11
Which amino acid is not essential in humans?
Glutamate
What is the common feature of ketogenic amino acids?
They can be converted to acetyl CoA or acetoacetate
Which amino acid is converted to cysteine?
Methionine
How many amino acids are essential in the human diet?
8
Study Notes
Amino Acids
- Amino acids have various color reactions with the properties of their specific groups, which is important in recognizing amino acids and thus proteins.
- Ninhydrin is a color reaction specific to all amino acids.
Peptides and Polypeptides
- Two amino acid molecules can be covalently joined through a substituted amide linkage, termed a peptide bond, to yield a dipeptide.
- When a few amino acids are joined in this fashion, the structure is called an oligopeptide, and when many amino acids are joined, the product is called a polypeptide.
Polypeptide Chain
- A polypeptide chain consists of amino acids joined by peptide bonds.
- The amino terminal of amino acid has a free amino group (NH3+) and the carboxy terminal has a free carboxyl group (COO-).
Amino Acids as Acids and Bases
- Each amino acid has a particular pH called the isoelectric point at which the overall charge on an amino acid molecule is zero.
- At the isoelectric point, an amino acid exists as a zwitterion, with the carboxyl group donating a proton to the amino group.
Acid-Base Properties
- Amino acids can act as acids or bases, depending on their R groups.
- At a pH more acidic than the isoelectric point, the amino acid forms a positive ion, and at a pH more alkaline than the isoelectric point, the amino acid forms a negative ion.
Classification of Amino Acids
- Amino acids can be classified into five main classes based on the properties of their R groups:
- Nonpolar, aliphatic R groups (e.g., alanine, valine, leucine, isoleucine)
- Aromatic R groups (e.g., phenylalanine, tyrosine)
- Negatively charged (acidic) R groups (e.g., aspartate, glutamate)
- Positively charged (basic) R groups (e.g., lysine, arginine, histidine)
- Uncommon R groups (e.g., hydroxyproline, γ-carboxyglutamate)
Uncommon Amino Acids
- Uncommon amino acids include 4-hydroxyproline, γ-carboxyglutamate, desmosine, and others.
Essential Amino Acids
- Essential amino acids must be taken in through the diet, as humans do not have the enzymes to synthesize them.
- Essential amino acids for humans include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Glycogenic and Ketogenic Amino Acids
- Glycogenic amino acids are converted to pyruvate, α-ketoglutarate, oxaloacetate, fumarate, or succinyl CoA.
- Ketogenic amino acids are converted to acetyl CoA or acetoacetate.
Learn about the clinical importance of color reactions in amino acids, their properties, and how to recognize them using Ninhydrin. This is crucial in understanding proteins.
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