Isoelectric & Protein structures

Isoelectric point

the pH at which the net charge on a protein becomes zero.

Below, above

____ pI, the molecule has a positive net charge; ____ pI, it has a negative net charge.

opposite charge

Proteins move toward the electrode with the _____.

zero

At pH = pI, positive and negative charges may still be present, but their algebraic sum is ____.

Electrophoretic mobility

_____ becomes approximately zero, and solubility is often - but not universally - lowest because electrostatic repulsion is minimized

electronic points

Proteins are separated in a pH gradient according to their _____

Electrophoresis

a laboratory technique used to separate DNA, RNA or protein molecules based on their size and electrical charge

protein purification

The isoelectric point is significant in _______ because it represents the pH where solubility is typically minimal

Acidic solution

______ is rich in H+, so the molecule accepts and keeps more protons.

Basic solution

______ favors loss of H+.

pI

exact budget at which the positive and negative charges balance

Acid-base reactions

_____ favor transfer from the stronger acid to the stronger base.

acid, base

An ____ donates H+; a ____ accepts H+.

alpha-carboxyl group, alpha-ammonium group

For a generic amino acid, the _____ is commonly the first group to deprotonate, while the ______ loses H+ at a higher pH.

pKa

the pH at which one ionizable group is 50% protonated and 50% deprotonated

Zwitterions

molecules that have both a positively charged part and a negatively charged part, but are still neutral overall.

zwitterionic form

The ______ has two point charges but has a net charge of 0

Amphoteric

can act as both an acid and a base

amino-acid carboxyl, amino

The _____ group donates H+ and becomes COO-, while the ____ group accepts H+ and becomes NH3+.

carboxyl

The ____ group acts as an acid by losing a hydrogen atom (negatively charged group), which is essentially just a proton.

Acidic pH, basic pH

_____ generally promotes protonation and a more positive charge; ______ promotes deprotonation and a more negative charge.

extremely low pH

At ________ (e.g. pH 1), both carboxylic acids should be protonated along with the amino group, for a net charge of +1.

Protonation

means that a species gains a proton (H+)

decreases

As acidity ______, the most acidic carboxylic acid will be deprotonated to give us the zwitterionic form (net charge = 0)

deprotonation

means that a species loses a proton

increase in basicity

With a further _______, the second carboxylic acid is deprotonated to give a net negatively charged form (net charge = -1)

very high basicity

At _______ (e.g. pH 14) the ammonium will be deprotonated, giving a doubly negatively charged species (net charge = -2)

titration curve

A ______ therefore contains buffering regions near each pKa and plateaus of predominant integer charge between transitions

peptide bonds

are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another

Proteins

macromolecules that perform a wide variety of functions in cells, from catalyzing reactions to providing structural support

Peptide Bonds

Covalent bonds that link amino acids together

dehydration synthesis

Peptide bond formation is a ____ reaction.

Primary Structure

consists of a sequence of amino acids linked together by peptide bonds and includes any disulfide bonds.

Primary Structure

This sequence is critical because it dictates the higher-level structures of the protein.

Primary structure

The peptide bonds between amino acids form this linear chain, which is the simplest level of protein structure.

Secondary Structure

refers to particularly stable arrangements of amino acid residues giving rise to recurring structural patterns.

Secondary Structure

In _____, small sections of the protein chain fold or twist due to hydrogen bonding between backbone atoms.

alpha helix, beta-pleated sheet

The _____ is a coiled structure, whereas the ______ consists of extended strands of the polypeptide chain that fold back onto themselves, forming sheet-like structures.

Tertiary Structure

the overall 3D folding of the entire polypeptide chain

Tertiary Structure

It forms when side chains (R-groups) interact with each other.

Tertiary Structure

This level of structure is stabilized by a variety of interactions, including hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interactions between the side chains (R groups) of the amino acids.

Disulfide bonds

covalent bonds formed between cysteine residues, also contribute to the stability of the tertiary structure

Quaternary Structure

occurs only in proteins made of more than one polypeptide chain.

Quaternary Structure

is essential for the functionality of multi-subunit proteins and allows for cooperative interactions

denatured

A protein becomes _____ when its normal shape gets deformed because some of the hydrogen bonds are broken.

Weak hydrogen bonds

_______ break when too much heat is applied or when they are exposed to an acid

inactive, easily digested. crystallized

Biological Changes

● Proteins become _____

● Denatured proteins are ______

● Denatured proteins cannot be ______

decreased solubility, maximum precipitation

Chemical Changes

● Proteins show _______ and

________

● Many chemical groups become inactive

Viscosity, diffusion

Physical Changes

● ______ of protein solution increases

● Rate of _____ decreases