Chapter 14 ProteinsSections: 1-4

By: Kelsi Myer and Rachel Dailey

Proteins- are by far the most important of all biological compounds The word protein is derived from the Greek word

proteios meaning “of first importance” Perform a variety of functions in the body

1. structure- proteins are the chief constituents of skin, hair, bones, and nails. Two important structural proteins

Collagen keratin

Section 1 What are the Many Functions of Proteins?

keratin

2. Catalysis- most all reactions in the body are catalyzed by enzymes 3. Movement- muscles are made of protein molecules

Every movement you make, blinking, pointing, walking breathing, can only happen because of the proteins

2 main Myosin Actin

4. Transport- movement of vital nutrients Hemoglobin carries oxygen to cells and carbon dioxide to lungs to be expelled

5. Hormones- many hormones are proteins Insulin Erythropoietin Human growth hormone

Functions continued

6. Protection- antibodies are proteins made to counteract proteins of a foreign nature. Major for fighting disease Blood clotting (Fibrinogen)

7. Storage- store materials Ferritin is found in liver and stores iron

8. Regulation-control expression of genes and dictate when manufacturing of other proteins take place

Functions Continued

An average call contains 9000 different proteins The human body had 100,000 different proteins Classified into two types

Fibrous proteins Insoluble in water Structural purposes

Globular proteins More or less soluble in water Used for nonstructural purposes

Proteins

There is a wide variety but they have basically the same structure: they are chains of amino acids. Contains an amino group and a carboxyl group

20 common amino acids in nature make up proteins They are called alpha amino acids All but one of the 20 fit the formula. It only differs

between the R and N bond. The most important aspect of the R groups is their polarity. Classified into four groups

Nonpolar ( hydrophobic; repel water) Polar but neutral (hydrophilic; attracts water) Acidic (Hydrophilic) Basic (hydrophilic)

Section 2 What are Amino Acids?

General protein formula

They are chiral with carbon stereocenters The exception is glycine which is achiral Each amino acid exists as two enantiomers

but nature only makes one of the two; usually the L- isomer

All the amino acids in your body are the L-isomer. D amino acids are extremely rare but can be found in the cell walls of some bacteria

Section 2 continued

Amines cannot exists in the presence of a moderately weak acid.

They gain a proton to form ammonium ions RNM3+

Has a –COOH group Has a –NH2 group Water insoluble Compounds with a positive charge on one atom

have a negative charge on another

Section 3: What are Zwitterions?

The negatively charged atom is called zwitterions From german zwitter

“hybrid” Amino acids are zwitterions in water solutions

and solids Ionic compounds Internal salts

Un-ionized ions do NOT exist

Zwitterions

All are solids Have high melting points 20 common are all fairly soluble in water If pH is lowered the zwitterion will turn to a positive ion

-COO gets proton If pH is raised the zwitterion will turn to negative ion

-NH2 gives proton to –OH Pretty close isoelectric points

Isoelectric point is where all the molecules have equal positive and negative charges

15 of 20 are around 6

Properties

Around isoelectric point amino acids exists in aqueous solution largely as zwitterions

React with either A strong base A strong acid

Proteins

The side chain of an amino acid is responsible for the unique characteristics of these molecules.

The amino acid cysteine has a chemical property not shared by the other 20 amino acids.

Cystine is the dimer of cysteine and can be reduced easily to give two molecule of cysteine.

Many amino acids have acidic or basic properties. Glutamic acid and aspartic acid and they have carboxyl

groups in their side chains, addition to the ones that amino acids all have. The carboxyl group can lose a proton and for the carboxylate

anion. Because of the carboxylate the side chains of the two amino

acids are negatively charged at neutral pH.

Section 4 What Determines the Characteristics of Amino Acids?

Histidine, lysine, and arginine have basic side chains Lysine and arginine are positively charged or near neutral pH

The pKa values for amino acids depend on the environment and change significantly within the confines of the protein.

Histidine can be found in the protonated or unprotonated forms in proteins and the properties of many proteins depends on whether individual residues are charged or not.

The charged amino acids are found in the active sites of enzymes.

Section 4 Continued

Phenylalanine, tryptophan, and tyrosine have aromatic rings in their side chains

They allow us to locate and measure proteins because the aromatic rings absorb strongly at 280 nm and can be detected using a spectrophotometer.

Important in physiology because they are precursors to neurotransmitters.

Tryptophan is converted to serotonin and has a calming effect Low levels are associated with depression High levels produce a manic state

Section 4 Continued

Tyrosine (derived from phenylalanine) is converted to the neurotransmitter class called catecholamines

Includes adrenalin L-Dihydroxyphenylalanine (L-dopa) is an intermediate

of tyrosine Associated with Parkinson’s disease Tyrosine supplements increase the levels of dopamine

but L-dopa is usually prescribed because it passes into the brain quickly through the blood-brain barrier.

Section 4 Continued

Tyrosine and phenylalanine are precursors to norepinephrine and epinephrine Both are stimulatory

Epinephrine is known as the “fight or flight” hormone Releases glucose and nutrients into the blood and

stimulates brain function. Tyrosine is said to give a morning lift Tryptophan helps sleep at night

Milk proteins have high levels of tryptophan

Section 4 continued