BL/CH 401 Lecture #4 -- Amino Acid Structures

THE POLAR AMINO ACIDS

Polar AAs are important since they provide chemical groups for interaction with water.

Thus, the hydrogen bonding character of polar AAs is key in forming protein structures.

While the ionic bonding character of charged polar AAs is also important in protein structure.

Also the polar side chains in these AAs provide the chemically reactive groups in proteins.

Alcohols - Neutral Polar Amino Acids

Polar -- Neutral -- Alcohols -- Ser (Serine) -- in computers Ser = S

{*Figure 12*}

Ser contains one -OH group and so it is essentially hydroxy-Ala.

The hydroxyl group on Ser does not normally ionize, so it is not charged in proteins - its neutral.

Ser is the smallest AA of the polar amino acids and is very polar.

The hydroxyl group on Ser provides enzymes a very good nucleophilic group for doing chemistry.

Another important function of Ser is to form esters with phosphate, making phopho-ester proteins.

Phosphorylation of proteins/enzymes is very important in regulation of activity.

Polar -- Neutral -- Alcohols -- Thr (Threonine) -- in computers Thr = T

{*Figure 13*}

Thr adds a Carbon on to Ser, which makes the hydroxyl group less accessible in Thr than Ser.

Thr serves more often in a structural role in proteins and is usually not as chemically active as Ser.

Thr can form esters with phosphoric acid and phospho-Thr is often found in proteins.

Polar -- Neutral -- Alcohols -- Tyr (Tyrosine) -- in computers Tyr = Y

{*Figure 14*}

Tyr is an aromatic alcohol and so it has both aromatic character and polar character.

The hydroxyl of Tyr is like the hydroxyl in phenol, so at high pH it can ionize.

Tyr can also form phospho-esters like Ser and Thr.

Phospho-Tyr is very important in proteins/enzymes involved in regulating the cycle cell.

©Wilbur H. Campbell, 1995, 1996; wcampbel@mtu.edu

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