Biochemistry Quiz on Amino Acids and Peptides

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Questions and Answers

What distinguishes selenocysteine from cysteine?

Amino acid with no specific function

Contains selenium instead of sulfur (correct)

Derived from cysteine instead of serine

Contains sulfur instead of selenium

Which amino acids serve as neurotransmitters or their precursors?

All amino acids without exception

Only L-α-amino acids

Some L-α-amino acids and their derivatives (correct)

Only non-protein amino acids

What role do peptides play in biological processes?

Act as hormones or neuromodulators (correct)

Exclusively as energy sources

Only as structural elements in proteins

Only as components of cell membranes

What defines the N-terminus of a polypeptide chain?

The start with a free amino group Signup and view all the answers

How are peptide bonds formed between amino acids?

By reacting the carboxyl group of one amino acid with the amine group of another Signup and view all the answers

Which amino acids have non-polar side chains?

Glycine, Tryptophan, Methionine Signup and view all the answers

What is true about amino acids with uncharged polar side chains?

They contain hydroxyl or amide groups that can form hydrogen bonds with water. Signup and view all the answers

What characterizes amino acids with basic side chains?

They include amino acids like Lysine, Arginine, and Histidine. Signup and view all the answers

Which functional group is characteristic of cysteine?

Sulfhydryl group Signup and view all the answers

What is the charge of aspartic acid's side chain at physiologic pH?

Negatively charged carboxylate group Signup and view all the answers

What is the role of the imidazole group in histidine?

It acts as a proton donor/acceptor in enzyme reactions. Signup and view all the answers

Proline is unique among amino acids because it contains:

A ring structure formed between its side chain and α-amino group Signup and view all the answers

Which amino acids can form disulfide bonds?

Cysteine only Signup and view all the answers

Which of the following amino acids is classified as both essential and semi-essential?

Arginine Signup and view all the answers

Which amino acid is exclusively ketogenic?

Leucine Signup and view all the answers

What is the primary function of glycogenic amino acids?

To serve as glucose precursors Signup and view all the answers

Which of the following is NOT a non-essential amino acid?

Leucine Signup and view all the answers

What distinguishes chiral amino acids from non-chiral amino acids?

The orientation of the α-carbon atom Signup and view all the answers

Which of the following amino acids is NOT categorized as glycogenic?

Lysine Signup and view all the answers

Which amino acid is derived from phenylalanine?

Tyrosine Signup and view all the answers

Which classification includes amino acids that can form both glucose and fat?

Glycogenic and ketogenic amino acids Signup and view all the answers

What is the configuration of all amino acids found in proteins?

L-configuration Signup and view all the answers

Which property is common to most amino acids regarding their solubility?

Soluble in water Signup and view all the answers

Which amino acid is an exception regarding optical isomers?

Glycine Signup and view all the answers

What is a characteristic of amino acids being ampholytes?

They contain both acidic and basic groups. Signup and view all the answers

Under what pH condition does an amino acid exist as a cation?

Acidic pH Signup and view all the answers

Which of the following amino acids is a derivative of common amino acids found in collagen?

4-hydroxyproline Signup and view all the answers

Which uncommon amino acid is associated with blood clotting proteins?

Gamma-carboxyglutamate Signup and view all the answers

What distinguishes selenocysteine from other amino acids?

It is incorporated during protein synthesis. Signup and view all the answers

What is the basic structural feature common to all proteins?

They are linear polymers of amino acids. Signup and view all the answers

Which of the following correctly describes the electrical charge of amino acids at physiological pH?

The amino group is protonated and the carboxyl group is dissociated. Signup and view all the answers

Which of the following amino acids is represented by the one-letter abbreviation 'H'?

Histidine Signup and view all the answers

What is NOT one of the classifications of amino acids mentioned?

Amino acids with basic side chains Signup and view all the answers

Which of the following correctly lists an example of a contractile protein?

Myosin Signup and view all the answers

What role do the side chains of amino acids play in proteins?

They dictate the amino acid's role in the protein. Signup and view all the answers

Which amino acid is represented by the 3-letter abbreviation 'Gly'?

Glycine Signup and view all the answers

Which of these statements about blood proteins is incorrect?

Immunoglobulins are involved in oxygen transport. Signup and view all the answers

Study Notes

Biochemistry Lecture 3

Proteins: Abundant and diverse molecules in living systems; essential for all life processes; composed of amino acids.
Example Proteins: Enzymes, polypeptide hormones, muscle protein myosin, collagen, hemoglobin, plasma albumin, immunoglobulins.
Protein Structure: Linear polymers of amino acids.

Amino Acids

Structure: Organic compounds with two functional groups: amino group (-NH2) which is basic, and carboxyl group (-COOH) which is acidic.
General Structure: Central carbon atom (α-carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain ("R-group").
Proline Exception: Proline's α-carbon is part of a ring structure, lacking a primary amino group.

Amino Acid Structure and Properties

At Physiological pH (approx. pH 7.4): Carboxyl group is dissociated as a carboxylate ion (-COO-), and the amino group is protonated (-NH3+).
Properties: These dictate the amino acid's role in proteins. Classification is based on properties of side chains.

Amino Acid Classification

Based on structure:
- Non-polar side chains (hydrophobic): Glycine, alanine, valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, proline; side chains cluster in protein interior.
- Uncharged polar side chains (hydrophilic): Serine, threonine, cysteine, tyrosine, asparagine and glutamine; these form hydrogen bonds with water; cysteine can form disulfide bonds.
- Acidic side chains: Aspartic and glutamic acids; fully ionized as negatively charged carboxylate ions at physiological pH.
- Basic side chains: Lysine, arginine, and histidine; their side chains are positively charged at physiological pH; lysine has a second amino group; arginine has a guanidine group; histidine has an imidazole group.

Nutritional Classification of Amino Acids

Essential amino acids: Cannot be formed in the body and must be obtained through the diet; crucial for growth, health, and protein synthesis (valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, lysine, histidine, threonine, arginine).
Semi-essential amino acids: Body can produce them, but not in quantities needed by growing children, must be supplied through diet (arginine and histidine).
Non-essential amino acids: Body can produce them in sufficient quantities for adults and children (alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine). Tyrosine is produced from phenylalanine.

Metabolic Fate of Amino Acids

Glycogenic amino acids: Their carbon skeletons are degraded to pyruvate, α-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate, subsequently converted to glucose. Include alanine, cysteine, glycine, Arginine, glutamine, isoleucine, tyrosine, etc.
Ketogenic amino acids: Their carbon skeletons form precursors for fat synthesis; examples include leucine, and lysine.
Glycogenic and ketogenic amino acids: Some amino acids serve as precursors for both glucose and fat (isoleucine, phenylalanine, tryptophan, tyrosine, and threonine).

Other Important Amino Acids

4-Hydroxyproline and 5-Hydroxylysine: Modified forms of proline and lysine, found in plant cell wall proteins and collagen.
γ-Carboxyglutamate: Found in blood clotting protein prothrombin and proteins binding Ca2+.
Desmosine: A derivative of four lysine residues, found in elastin.
Selenocysteine: A 21st amino acid; derived from serine, and found in few known proteins.
Ornithine and citrulline: Not protein constituents but are involved in various metabolic pathways.

Function of Amino Acids

Protein synthesis: Providing monomers for polypeptide chains.
Neurotransmitters and hormones: Acting as neurotransmitters, mediators, or hormones, or precursors to them.
Precursors for metabolites: Forming precursors for glucose, porphyrins, purines, pyrimidines, and urea.
Components of other molecules: Participating in lipid synthesis (serine in phospholipids, glycine in bile salts) and in polypeptide antibiotics.

Peptide Chain

Peptides: Short chains of amino acids linked by peptide bonds.
Polypeptides: Long chains of amino acids; N-terminus is the starting point, and C-terminus is the ending point.

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Description

Test your knowledge on the key differences between selenocysteine and cysteine, the role of amino acids as neurotransmitters, and the significance of peptides in biological functions. Understand the structural aspects of polypeptides, including the formation of peptide bonds and the definition of the N-terminus. This quiz covers essential concepts in biochemistry.