Protiens and Amino Acids 2

Transcription

1 Protiens and Amino Acids 2 By Alaa J. Mahrath, Biochemistry Department / College of Medicine/ Babylon University

2 2 Amino Acid Stereoisomers Protiens and Amino Acids All the a-amino acids except for glycine are chiral because the a-carbon is attached to four different atoms. Thus amino acids can exist as D and L stereoisomers. We can draw Fischer projections for a-amino acids by placing the carboxylate group at the top and the R group at the bottom. In the L stereoisomer, the NH 3 + group is on the left, and in the D stereoisomer, the NH 3 + group is on the right. In biological systems, the only amino acids incorporated into proteins are the L stereoisomers. There are D amino acids found in nature, but not in proteins. Let s take a look at the stereoisomers for L- and D-glyceraldehyde, a carbohydrate, and two amino acids, L- and D-alanine, and L- and D-cysteine.

3 3 In amino acids, the NH 3 + group appears on the right or left of the chiral carbon to give D or L stereoisomers. Protiens and Amino Acids

4 4 Essential Amino Acids: Protiens and Amino Acids 20 amino acids used to build the proteins in the body, only 11 can be synthesized in the body. The other 9 amino acids, listed in Table.3, are essential amino acids that must be obtained from the proteins in the diet. Table 3: Essential Amino Acids for Adults

5 5 Complete proteins, which contain all of the essential amino acids, are found in most animal products such as eggs, milk, meat, fish, and poultry. However, gelatin and plant proteins such as grains, beans, and nuts are incomplete proteins because they are deficient in one or more of the essential amino acid. Figure 2 :Complete proteins such as eggs, milk, meat, and fish contain all of the essential amino acids. Incomplete proteins from plants such as grains, beans, and nuts are deficient in one or more essential amino acids

6 6 Protiens and Amino Acids Table 4: Amino Acid Deficiencies in Selected Vegetables and Grains

7 7 Amino Acids as Acids and Bases learning Goal : Draw the condensed structural formula for an amino acid at ph values above or below its isoelectric point. amino acid can exist as a positive ion if a solution is more acidic (has a lower ph) than its pi or as a negative ion if a solution is more basic (has a higher ph) than its pi. Remember the pi values for nonpolar and polar neutral amino acids are from ph 5.1 to 6.3. Example : take a look at the different ionic forms of alanine. Protiens and Amino Acids alanine has a zero overall charge at its pi of 6.0 with a carboxylate anion ( COO - ) and an ammonium cation ( NH 3+ ) However, the charge balance changes when alanine is placed in a solution that has a ph that is more acidic or more basic than its pi.

8 8 Amino acid as Acid Protiens and Amino Acids In a solution that is more acidic, with a lower ph than its pi (ph < 6.0), the COO- group of alanine gains H+, which forms its carboxylic acid ( COOH). Because the NH 3 + group retains a charge of 1+, alanine has an overall positive charge (1+) at a ph lower than 6.0.

9 9 Amino Acid as base Protiens and Amino Acids When alanine is in a solution that is more basic, with a higher ph than its pi ( ph > 6.0), the NH 3 + group loses H + and forms an amino group ( NH 2 ), which has no charge. Because the COO- group has a charge of 1-, alanine has an overall negative charge (1-) at a ph higher than 6.0. The changes in the ionized groups with ph above, below, or equal to the pi are summarized in Table 5.

10 10 Table5:Ionized Forms of Nonpolar and Polar Neutral Amino Acids The pi values of the acidic amino acids (aspartic acid, glutamic acid) are around ph 3. At ph values of 3, the carboxylic acid group in the R groups is not ionized. However, at physiological ph values, which are greater than 3, the carboxylic acid in the R group loses H+ to form a negatively charged carboxylate group COO-.

11 11 While the pi values of basic amino acids are typically higher than physiological ph values, ranging from ph 7.6 to Thus at physiological ph values, the amines in the R groups of the basic amino acids (lysine, arginine, and histidine) gain H+ to form positively charged ammonium groups NH 3 +. Example : The pi of glycine is 6.0. Draw the condensed structural formulas for glycine at : A. ph 6.0 and at B. ph 8.0. State the overall charge for each.

12 STUDy CHECK : (H.W) Draw the condensed structural formula for glycine at ph 3.0 and give the overall charge. 12

13 السالم عليكن ورمحة اهلل وبركاته Thank You for your Attention 13