A Preliminary Introduction to the Nature of Phenylalanine and Some

Basic Reactions Related to It

Xiaoyue Shi

St John’s College Cardiff, Cardiff, CF3 5YX, U.K.

Keywords:

Phenylalanine, Biosynthesis, Metabolism Route, Phenylketonuria.

Abstract: The primary object of this study is to survey the nature of phenylalanine, and to study some basic reactions

or other information related to it. This is a working which is based on theoretical research. Detailed

information has been acquired about dehydration and condensation reaction, ionisation in water, important

metabolisms, or a disease called phenylketonuria and so on. The work has contributed to our present

understanding of this aromatic amino acid, and is a good reference for beginners in Chemistry who are

interested in this topic.

1 INTRODUCTION

In recent years, with the deepening of biochemical

research, more and more properties of amino acids

have been recognized by people.

Phenylalanine(C

), with the bonding

structure shown in Figure 1, as an aromatic amino

acid and one of the essential amino acids that human

bodies need, is also being developed in more and

more fields, such as biosynthesis, microbio

fermentation, treatment of relevant diseases.

Figure 1: The Bonding Structure of Phenylalanine.

Unlike those professional researches, this article

will provide an introduction to phenylalanine by

giving a dossier of important information and facts.

In the work, we introduce phenylalanine from four

main aspects: the mode of obtaining, the contrast of

solubility, basic reactions related and the metabolism

in the human body. This work is a good source of

useful information for those who are just beginning

to do research in relavant field.

2 HOW IS PHENYLALANINE

OBTAINED?

2.1 In Nature

Figure 2 shows the biosynthesis of phenylalanine.

Aromatic amino acid, which includes phenylalanine,

tyrosine, and tryptophan, can only be produced by

plants and microbes. There is a common path in their

synthesis, and shikimic acid is a common precursor

to the synthesis of these aromatic acid. In this case we

can call the common pathway shikimic acid pathway,

that is, shikimic acid as the starting material until the

formation of chorismic acid. The aromatic amino acid

production pathway branches at chorismic acid.

There are two pathways after this branch point: one

to generate phenylalanine and tyrosine, and the other

to form tryptophan. Chorismic acid is transformed to

prephenic acid by chorismate mutase, which is then

dehydrated and decarboxylated to form

phenylpyruvic acid for phenylalanine production

(Xiao, 2014).

From phenylpyruvic acid to L-phenylalanine, the

molecules react under transamination. The

mechanisms are shown in Figure 3.

138

Shi, X.

A Preliminary Introduction to the Nature of Phenylalanine and Some Basic Reactions Related to It.

DOI: 10.5220/0012003700003625

In Proceedings of the 1st International Conference on Food Science and Biotechnology (FSB 2022), pages 138-142

ISBN: 978-989-758-638-5

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

Figure 2: The Biosynthesis of Phenylalanine

Figure 3: The Mechanisms for Synthesis of Phenylalanine (Hu, 1985).

2.2 None-Nature

Chemical synthesis, enzymatic technique, microbial

fermentation, and asymmetric hydrogenation are the

four primary approaches of non-natural

phenylalanine synthesis.

As for chemical synthesis, somebody has already

done it successfully as early as 1882. However, the

route was too long and expensive.

For enzymatic method, there are two ways:

Transaminase is used to make phenylalanine from

phenylpyruvic acid, or phenylalanine ammonia-lyase

is used to make phenylalanine from cinnamic acid.

There are several advantages such as higher product

concentration, fewer steps, and stronger production

capacity. However, the cost of raw materials is

excessive.

For microbial fermentation, there are two types:

Precursor fermentation and direct fermentation. The

former uses some intermediates in the pathway of

amino acid biosynthesis as raw materials for

fermentation to produce amino acids. Unfortunately,

the cost is too high. In the latter, amino acids are

produced directly from inorganic carbon and nitrogen

sources, which is cheap and easy to obtain.

Furthermore, because the reaction may be performed

at normal temperature and pressure, this is one of the

most common methods for producing amino acids

(Lu, 2011; Pan, 1996).

For asymmetric hydrogenation, it adds a molecule

of hydrogen across the double bond between α and β

carbon of the amino acid. This is a method that people

use extensively. This work was also recognised with

the Nobel Prize in Chemistry (Li, 2002).

A Preliminary Introduction to the Nature of Phenylalanine and Some Basic Reactions Related to It

139

Figure 4: Solubility of 20 Amino Acids.

Figure 5: The Dehydration and Condensation Reaction of Two Amino Acid Molecules.

3 COMPARISON OF

SOLUBILITY OF 20 AMINO

ACIDS

Figure 4 shows the solubility of these amino acids,

which is one measure of their characteristics. Even

though there are only 20 of them, because they have

such a wide range of properties, when starting to

combine them, an infinite number of possibilities can

be made which leads to quite a considerable number

of differences in properties. Nature is guiding

evolution by property from an evolutionary

standpoint. Whatever the thing is, it needs to do

something more efficiently and more effectively so

that it can survive. And the fact that it has so many

options feeds into this evolutionary development,

which is beneficial to nature. For instance, nature

finds a way to resist to medicine over time. The

organism is trying to overcome whatever fighting.

Often, it is just a single amino acid reaction.

4 SOME BASIC REACTIONS

4.1

The Dehydration and Condensation

Reaction of Two Amino Acid

Molecules

This is a reaction that join two amino acid molecules

together. The general formula of reaction and the

reaction of phenylalanine are shown in Figure 5.

The condensation reaction between two amino

acid molecules is known as amino acid dehydration

and condensation. Amino and carboxyl groups also

generate a water molecule together.

The bonds that bind two amino acid molecules are

called peptide bonds. A compound which is formed

by the dehydration and condensation of two amino

acid molecules is called a dipeptide, which contains a

peptide bond. This process can be performed

hundreds of thousands of times to produce a molecule

that is extremely large. A polypeptide is a substance

that is generated by the dehydration and condensation

of several amino acid molecules and contains

multiple peptide linkages. A peptide chain is a type of

polypeptide that has a chain structure. A protein

molecule with a specific spatial structure can be

formed by twisting and folding peptide chains. A cell

contains hundreds of thousands of amino acids of

various kinds. The order of amino acids varies greatly

FSB 2022 - The International Conference on Food Science and Biotechnology

140

Figure 6: Reactions for Ionization of Phenylalanine in Water.

Figure 7: The Concentration of Different Ions in the Process of Phenylalanine Ionization (Fu, 2010).

when forming a peptide chain, and the way of

twisting, folding, and forming the spatial structure

also differs in thousands of ways. As a result, protein

molecules have a wide range of structures (Chen,

2015).

4.2

Ionization of Phenylalanine in

Water

The presence of R

and R

COOH in amino acids

makes them amphoteric. According to the quantity of

amino and carboxyl groups linked, all amino acids

can be categorised into three types: neutral amino

acid, acidic amino acid, and basic amino acid.

For phenylalanine, it is a neutral amino acid.

Figure 6 shows the two steps of ionization.

When pH < pKa

, the main form in solution is

COOH.

When pH = pKa

, there is about 50% of

COOH and 50% of H

COO

When pKa

< pH < pKa

, the main form in

solution is H

N+C

COO

When pH = pKa

, there is about 50% of

H3N

COO

and 50% of H

COO

When pH > pKa

, the main form in solution is

COO

(Qie, 2000).

This can be converted into a graph, as shown in

Figure 7.

5 METABOLISM IN THE HUMAN

BODY AND A RELATED

DISEASE

5.1

Metabolism Route

Figure 8 shows the metabolism route of

phenylalanine. Inside the human body, most of the

phenylalanine are oxidized to tyrosine by the

catalysis of phenylalanine hydroxylase, and together

with tyrosine, they synthesize important

neurotransmitters and hormones, and participate in

the body’s glucose and fat metabolism. The

remaining phenylalanine is converted into

phenylpyruvic acid.

A Preliminary Introduction to the Nature of Phenylalanine and Some Basic Reactions Related to It

141

Figure 8: The Metabolism Route of Phenylalanine.

5.2 The Disease: Phenylketonuria

Mutations in any of the genes that encode the

enzymes needed can lead to defects in the activity of

those enzymes, which may lead to metabolic

disorders in phenylalanine.

There are two types of phenylketonuria:

deficiency of PKU or deficiency of BH

Phenylalanine cannot be turned into tyrosine in the

former, resulting in brain cell destruction. However,

the latter not only makes phenylalanine unable to be

converted into tyrosine, it also blocks important

neurotransmitters and therefore exacerbates the

damage to the nervous system. When

phenylketonuria patients are born, they normally

have no abnormalities and are easy to be

misdiagnosed (Zhang, 2022). Symptoms normally

appear between the ages of 3 and 6 months, and they

are most noticeable at the age of one year.

Phenylketonuria patients should avoid

phenylalanine-containing foods.

Phenylalanine (C

) can form

phenylpyruvic acid (C

) under transamination,

which can be further converted into derivatives such

as phenylacetic acid (C

). At this point, the urine

is full of various metabolites including big amount of

phenylpyruvic acid. That is why the disease is called

phenylketonuria (Xie, 2011).

6 CONCLUSION

In this investigation, the aim is to provide basic

information and important facts of phenylalanine for

beginners on this topic, which includes the way of

obtaining in both nature and non-nature, the

comparison of solubility of 20 amino acids, two

typical reactions and its activity in the human body. It

is unfortunate that the study did not include any in-

depth knowledge, as they are too advanced for

beginners. The insights gained from this study may

be of assistance to future research into certain aspect

of phenylalanine. This would be a fruitful area for

further work.

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