Structure & Function of Amino Acids & Proteins

Comprehensive vocabulary flashcards covering the structure, classifications, metabolic fates, biological functions, and clinical correlations of amino acids and proteins.

Amphoteric

The property of amino acids that allows them to act as either an acid or a base depending on the pH of the surrounding environment.

Zwitterion

The dipolar form of an amino acid occurring at its isoelectric point ($\text{pI}$), in which both the amino and carboxyl groups are ionized, resulting in a net electric charge of zero.

$\alpha$-Carbon

The central chiral center of an amino acid (in the L-isomer configuration) bonded to an amino group ($\text{-NH}_3^+$), a carboxyl group ($\text{-COO}^-$), a hydrogen atom, and a variable side chain (R-group).

Proline

A secondary amino acid containing a cyclic imino group that induces $\beta$-turns in polypeptide chains and provides structural stability to collagen.

Peptide Bond

A rigid, covalent amide linkage formed by a dehydration synthesis reaction that releases $\text{H}_2\text{O}$ between the $\alpha$-carboxyl group of one amino acid and the $\alpha$-amino group of the next, synthesized in the direction from N-terminal to C-terminal.

Essential Amino Acids

Amino acids that cannot be synthesized by the human body and must be obtained directly from the diet, such as Phenylalanine, Valine, and Tryptophan.

Non-Essential Amino Acids

Amino acids that can be synthesized endogenously by the body, such as Alanine and Aspartate.

Semi-Essential Amino Acids

Amino acids that are synthesized by the body but become nutritionally required in the diet during periods of rapid growth, specifically Arginine and Histidine.

Glucogenic Amino Acids

Amino acids whose carbon skeletons are degraded to yield pyruvate or citric acid cycle intermediates that serve as precursors for glucose production.

Ketogenic Amino Acids

Amino acids whose carbon skeletons are metabolized into acetoacetate or Acetyl CoA to serve as precursors for lipids and ketone bodies, exclusively exemplified by Leucine and Lysine.

Nonpolar Amino Acids

Hydrophobic, lipid-like amino acids that cluster in the interior core of folded proteins away from water; examples include Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Methionine, and Proline.

Uncharged Polar Amino Acids

Hydrophilic amino acids possessing side chains with a zero net charge at neutral pH that participate in hydrogen bonding; examples include Serine, Threonine, Tyrosine, Asparagine, Glutamine, and Cysteine.

Acidic Amino Acids

Negatively charged amino acids that act as proton donors and are fully ionized at physiologic pH, consisting of Aspartate (aspartic acid) and Glutamate (glutamic acid).

Basic Amino Acids

Positively charged amino acids that act as proton acceptors at physiologic pH, consisting of Histidine, Lysine, and Arginine.

Primary Structure

The linear sequence of amino acids linked by covalent peptide bonds, which encodes all necessary information to dictate higher-order folding and ultimate biological function.

Secondary Structure

Localized, regular conformational folding of the polypeptide backbone stabilized exclusively by hydrogen bonds between peptide bond atoms, giving rise to $\alpha$-helices and $\beta$-pleated sheets.

Tertiary Structure

The overall three-dimensional folding of a single polypeptide chain into its native, biologically active conformation, driven by hydrophobic interactions, hydrogen bonds, ionic bonds, and covalent disulfide cross-links between R-groups.

Quaternary Structure

The spatial arrangement and assembly of two or more individual polypeptide chains (subunits) stabilized by noncovalent interactions (hydrogen, ionic, and hydrophobic bonds), exemplified by the tetrameric structure of hemoglobin.

Disulfide Bond

A strong covalent cross-link formed between the sulfhydryl ($\text{-SH}$) groups of two Cysteine residues that stabilizes tertiary protein structure.

Protein Denaturation

The unfolding and loss of a protein's secondary, tertiary, and quaternary structures caused by heat, extreme pH, or organic solvents, disrupting non-covalent interactions while leaving covalent peptide bonds intact.

Protein Renaturation

The process by which an unfolded or denatured protein refolds into its functional three-dimensional native state upon removal of denaturing agents, guided by the primary amino acid sequence.

Chaperone Proteins

Specialized cellular proteins required by complex polypeptides to facilitate correct folding and prevent improper aggregation during protein folding or renaturation.

Sickle Cell Disease

A hemoglobinopathy caused by a missense mutation in the $\beta$-globin chain where polar Glutamate at position 6 is replaced by nonpolar Valine ($\text{Glu6Val}$), promoting abnormal quaternary polymerization into sickled hemoglobin ($\text{HbS}$).

Alanine (Ala)

A nonpolar amino acid functioning in the glucose-alanine cycle and transaminated by alanine aminotransferase (ALT), a clinical biomarker for liver injury.

Arginine (Arg)

A basic, semi-essential amino acid that serves as the biological precursor to nitric oxide, which mediates endothelial vasodilation.

Asparagine (Asn)

An uncharged polar amino acid that serves as a target site for N-linked glycosylation, therapeutically depleted by asparaginase chemotherapy.

Aspartic Acid (Asp)

An acidic amino acid that serves as an intermediate in the urea cycle and is transaminated by aspartate aminotransferase (AST), a clinical marker of cardiac injury.

Cysteine (Cys)

A sulfur-containing polar amino acid and precursor to glutathione whose toxicity depletion during acetaminophen overdose is managed with N-acetylcysteine (NAC).

Glutamine (Gln)

An uncharged polar amino acid that functions as the primary circulating carrier of ammonia and acts as an essential metabolic fuel during sepsis and trauma.

Glutamic Acid (Glu)

An acidic amino acid that acts as the primary excitatory neurotransmitter in the central nervous system and serves as the precursor for GABA synthesis.

Glycine (Gly)

The smallest amino acid, containing a hydrogen atom as its side chain; it acts as a precursor for heme and collagen synthesis and is mutated in osteogenesis imperfecta.

Histidine (His)

A basic amino acid that serves as the precursor to histamine, a critical mediator of Type 1 hypersensitivity allergic reactions and anaphylaxis.

Isoleucine (Ile)

A hydrophobic branched-chain amino acid (BCAA) that pathologically accumulates in Maple Syrup Urine Disease (MSUD).

Leucine (Leu)

A branched-chain amino acid that functions as a potent activator of the mTOR signaling pathway to stimulate muscle protein synthesis.

Lysine (Lys)

A basic essential amino acid subject to collagen cross-linking after being hydroxylated by a vitamin C-dependent hydroxylase enzyme.

Methionine (Met)

A sulfur-containing nonpolar amino acid that serves as the initiator amino acid during eukaryotic translation and is metabolically converted to homocysteine.

Phenylalanine (Phe)

An essential aromatic amino acid converted into tyrosine; impaired metabolism of this residue results in Phenylketonuria (PKU).

Tryptophan (Trp)

An essential aromatic amino acid that serves as the precursor to serotonin; its intestinal and renal transport is defective in Hartnup disease.

Tyrosine (Tyr)

A polar amino acid synthesized from phenylalanine that acts as a precursor to dopamine; its impaired enzymatic processing via tyrosinase deficiency results in albinism.

Catalytic Proteins

Enzymes, such as hexokinase and amylase, that accelerate biochemical reaction rates using specific active site R-group charges to bind substrates.