amino acids Flashcards
The citrulline isolated from watermelons is a D- or L-amino acid?
D- amino acid
the amino group placed at the right
How Much Alanine Is Present as the Completely
Uncharged Species?
At a pH equal to the isoelectric point
of alanine, the net charge on alanine is zero. Two structures can be drawn that have a net charge of zero, but the predominant form of alanine at its pI is zwitterionic
Why is alanine predominantly zwitterionic rather
than completely uncharged at its pI?
pI > pKa of the α-carboxyl group and pI < pKa of the α-amino group, so both groups are charged (ionized)
What fraction of alanine is in the completely uncharged form at its pI?
1 in 2.19 × 107. The pI of alanine is 6.01. From Table 3-1 and the Henderson-Hasselbalch equation, 1/4,680 carboxyl groups and 1/4,680 amino groups are uncharged. The fraction of alanine molecules with both groups uncharged is the product of these fractions.
How many chiral centers does isoleucine have?
2, Isoleucine is one of the two proteinogenic amino acids which contain a second chiral carbon atom beside the amino group-bearing α-C
How many optical isomers does isoleucine have?
4, there are a total of two chiral centres in isoleucine at the second and third carbon atoms. Since, there are two chiral centres, we will use our formula to find out the number of optical isomers. Hence, there are a total of 4 optical isomers of isoleucine.
Why does the value of pK1 increase with each
addition of an Ala residue to the Ala oligopeptide?
On addition of Ala residue to the oligopeptide the electrostatic interaction between the amino and carboxylic groups decreases (the non polar chain between the two increases). This decrease in the electrostatic interaction lowers the strength of acidic group (carboxylic group) resulting in the increase in the pK1 value.
Why does the value of pK2 decrease with each addition of an Ala residue to the Ala oligopeptide?
On addition of Ala residue to the oligopeptide the electrostatic interaction between the amino and carboxylic groups decreases (the non polar chain between the two increases). This decrease in the electrostatic interaction lowers the strength of basic group (amino group) resulting in the decrease in the pK2 value [pKb decreases with decrease in basic strength]
What is the approximate molecular weight of a peptide made up of 10 amino acids, provided the molecular weight of one amino acid is 128?
If we have 10 residues which are making the peptide then, in that case, the total molecular weight of the peptide after releasing the water molecule will be 12810-(918)
= 1280- 162
=1118 Dalton
two amino acid forms one peptide bond and releases one water molecule. We can also say that water molecules are released in peptide bonds are equal to the number of a peptide bond in the protein. Peptide of 10 amino acids will have 9 peptide bond and in this process, 9 water molecules are released
What pI would you predict for the pepsin proteins?
Pepsin has a very low pI in the range of 2-3 pH units, which is due to the high proportion of carboxyl residues, there is a total of 43. The protein is phosphorylated at Ser68 and has three disulphide bridges.
Which of each pair of the
polypeptides that follow is more soluble at the indicated pH.
•(a) (Phe-Met)3 or (Glu)20 at pH 7.0
•(b) (Lys-Ala)3 or(Gly)20 at pH 7.0
•(c) (Ala-Ser-Gly)5 or (Asn-Ser-His)5
at pH 6.0
•(d) (Ala-Asp-Gly)5 or (Asn-Ser-His)5 at pH 3.0
Polypeptides with polar or charged side chains are more soluble than polypeptides with nonpolar side chains.
(a) (Glu)20 is negatively charged at pH 7. (Phe-Met)3 is much less polar and hence less soluble.
(b) (Lys-Ala)3 is positively charged at pH 7. (Gly)20 is uncharged except for the amino- and carboxyl-terminal groups.
(c) At pH 6.0, (Asn-Ser-His)5 has polar Asn side chains and partially protonated His side chains.
(d) At pH 3.0, the carboxylate groups of Asp residues are partially protonated and neutral, whereas the imidazole groups of His residues are fully protonated and positively charged.
How many distinct dipeptides can be made from the two amino acids leucine and histidine? (Each amino acid may be used more than once.)
The order of the amino acids matters. For example, the peptide gly-ala is different from the peptide ala-gly. For each position in the peptide you must consider the number of possible amino acids that can be placed at that position. In other words, raise the number of different amino acids to the power of the length.
Number^(Length)
In this case, you have two different amino acids available (number = 2) for every position in the dipeptide (length = 2).
Number^(length)=2^(2)=4
determine the amino acid sequence
of the opioid leucine enkephalin
Leu-enkephalin is an endogenous opioid peptide neurotransmitter with the amino acid sequence Tyr-Gly-Gly-Phe-Leu that is found naturally in the brains of many animals, including humans. It is one of the two forms of enkephalin; the other is met-enkephalin
