Proteins and Amino Acids (Chapter 16) Lecture Flashcards

These flashcards cover the key vocabulary and concepts regarding the structure, function, and classification of proteins and amino acids, as well as enzyme kinetics and inhibition.

Proteins

Prevalent molecules in living organisms that are polymers made from 20 different amino acids.

$\alpha$-carbon

The central carbon atom of an amino acid bonded to an ammonium group ($-NH_3^+$), a carboxylate group ($-COO^-$), a hydrogen atom, and an R group.

Nonpolar Amino Acids

Hydrophobic amino acids with hydrocarbon side chains where the R group is H, alkyl, or aromatic.

Polar Amino Acids

Hydrophilic amino acids with R groups that are a hydroxyl, a thiol, or an amide.

Acidic Amino Acids

Amino acids where the R group is a carboxylate.

Basic Amino Acids

Amino acids where the R group is an amine, which ionizes to give an ammonium ion.

Zwitterion

A molecule or ion that has separate positively and negatively charged groups, characteristic of amino acids.

Isoelectric Point

The pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean.

Peptide Bond

An amide bond that forms when the $-COO^-$ group of one amino acid reacts with the $-NH_3^+$ group of the next amino acid.

N-terminus

The end of a peptide chain with the exposed ammonium group.

C-terminus

The end of a peptide chain with the exposed carboxylate group.

Essential Amino Acids

Nine amino acids that cannot be synthesized in the body and must be obtained from proteins in the diet.

Complete Proteins

Proteins from animal sources like eggs, milk, meat, and fish that contain all essential amino acids.

Primary Structure

The particular sequence of amino acids in a polypeptide chain held together by peptide bonds.

Alpha Helix ($\alpha$ helix)

A secondary structure where hydrogen bonds form between the oxygen of the $C=O$ group and the hydrogen of the $N-H$ group in the next turn of a spiral.

Beta-Pleated Sheet ($\beta$-pleated sheet)

A secondary structure where hydrogen bonds between carbonyl oxygen atoms and amide hydrogen atoms bend the polypeptide chain into a sheet.

Triple Helix

A secondary structure where three polypeptide chains are woven together and held by hydrogen bonds, providing strength to collagen and connective tissues.

Tertiary Structure

The overall three-dimensional shape of a protein determined by interactions such as hydrophobic/hydrophilic effects, salt bridges, hydrogen bonds, and disulfide bonds.

Disulfide Bonds

$-S-S-$ bonds between the $-SH$ groups of cysteine amino acids that stabilize tertiary and quaternary structures.

Quaternary Structure

The structural level consisting of the combination of two or more protein units, such as the four polypeptide chains in hemoglobin.

Denaturation

The disruption of bonds in the secondary, tertiary, and quaternary protein structures by heat, acids, bases, organic compounds, heavy metal ions, or agitation.

Enzymes

Proteins that act as biological catalysts, increasing the rate of reaction by lowering the energy of activation.

Active Site

A specific region within an enzyme that fits the shape of the substrate and catalyzes the reaction.

Lock-and-Key Model

A static model of enzyme action where the active site has a rigid shape that only binds substrates fitting exactly like a key.

Induced-Fit Model

A dynamic model of enzyme action where the enzyme structure is flexible and adjusts its active site shape to bind the substrate.

Isoenzymes

Different forms of an enzyme that catalyze the same reaction in different tissues but have quaternary structures with slight variations in amino acid sequences.

Competitive Inhibitor

A molecule with a structure similar to the substrate that competes for the active site; its effects can be reversed by increasing substrate concentration.

Noncompetitive Inhibitor

A molecule that binds to an enzyme away from the active site, changing the enzyme's shape and preventing substrate binding; not reversed by adding more substrate.

Irreversible Inhibitor

A molecule that forms a covalent bond with the enzyme, causing a permanent loss of catalytic activity.