Biochemistry Chapter 20: Proteins

Study Notes

General Characteristics of Proteins

Most abundant substances found in nearly all cells.
Composed of elements: carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
Naturally occurring unbranched polymers made of amino acids as monomer units.

General Characteristics of Amino Acids

Organic compounds featuring both an amino group (-NH2) and a carboxyl group (-COOH).
Found exclusively in protein structures, always in alpha form.
Alpha configuration arises from the amino group being attached to the central carbon (second carbon).

Importance of R-Groups

The R-group distinguishes different amino acids and is crucial for their classification.

Classifications of Amino Acids

Nonpolar (Hydrophobic): R-group mainly consists of hydrocarbons.
Polar (Hydrophilic): Classification depends on the presence of acidic, basic, or neutral characteristics:
- Acidic: extra -COOH group.
- Basic: contains an amino group.
- Neutral: includes -OH, -SH, and amide groups.

Properties of Amino Acids

Chirality: Most amino acids are chiral due to four different groups attached to the carbon center; glycine is an exception (not chiral).
Acid-Base Nature: Amino acids exist as charged species in solid and solution states.
Zwitterion and Isoelectric Point:
- Zwitterion: a molecule with both positive and negative charges, overall neutral.
- Isoelectric point: pH at which amino acids exist primarily in zwitterionic form (overall charge is zero).
- Variable pH ranges for different amino acids:
  - Nonpolar + polar neutral: 4.8-6.3
  - Polar acidic: < 3.2
  - Polar basic: > 7.5.
Electrophoresis: Process where separate charged particles migrate towards oppositely charged electrodes.

Cysteine - Unique Amino Acid

Only amino acid with a sulfhydryl group (-SH) in its side chain.
Categorized as a polar neutral amino acid.
Can dimerize in the presence of mild oxidizing agents to form cystine linked by disulfide bonds.
Disulfide linkage can revert to -SH groups with a reducing agent.

Peptides

Amino acids can bond to form unbranched chains.
Definitions vary by the number of amino acids:
- Dipeptide: 2 amino acids
- Tripeptide: 3 amino acids
- Oligopeptide: 10-20 amino acids
- Polypeptide: many amino acids.
Peptide backbone excludes R-groups; chains read from N-terminus to C-terminus.
Peptide bonds are covalent bonds formed between carboxyl and amino groups of amino acids (amine linkages).

Biochemically Important Peptides

Hormones:
- Oxytocin: regulates uterine contractions and lactation.
- Vasopressin: regulates kidney water excretion and affects blood pressure.
Neurotransmitters:
- Enkephalins: pentapeptides that reduce pain produced by the brain.
Antioxidants:
- Glutathione: a tripeptide regulating oxidation-reduction reactions.

General Structural Characteristics of Proteins

Proteins require a minimum of 40 amino acids to form functional structures.

Description

Explore the crucial characteristics of proteins and amino acids in this quiz based on Chapter 20 of Biochemistry. Understand the fundamental role proteins play in biological systems and their composition. Test your knowledge on the building blocks and properties of proteins.