Peptide bonds from a scientific perspective

Peptides (proteins) are present in every living cell and exhibit a wide range of biochemical activities. Some peptides are synthesized in the ribosomes of the cell through the translation of mRNA (messenger RNA), for example into hormones and signaling molecules. Other peptides are assembled (rather than synthesized) and become enzymes with a large number of different functions. Peptides also form the structure of receptors to which hormones and signaling molecules bind.

A peptide is a molecule created by linking two or more amino acids. In general, if the number of amino acids is fewer than fifty, these molecules are called peptides, while larger sequences are called proteins.

Thus, peptides can be considered small proteins. They are simply chains of amino acids.

Raw Components of Peptides (Amino Acids)

Amino acids are small molecules composed of atoms. Their structure includes a group made of a nitrogen atom (N) attached to two hydrogen atoms (H). This group is called the amino group and is written as (NH2).

In addition, their structure contains a second group made of a carbon atom (C) bound to two oxygen atoms (O) and one hydrogen atom (H). This group is called the carboxyl group and is written as (COOH).

Between these two groups there are atoms and bonds that are unique for each amino acid. In other words, all amino acids have two end groups formed by these groups (amino and carboxyl), between which lies a unique set of atoms.

Amino Acids

There are twenty standard amino acids in the human body that cells use to biosynthesize peptides (i.e., formation of peptides inside cells from amino acids). Our genetic code determines how peptides and proteins are synthesized from these amino acids.

Amino acids are divided into two groups: essential and non-essential amino acids.

• Essential amino acids are indispensable amino acids that the body cannot produce on its own and must be supplied through the diet. These include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Histidine is considered semi-essential, because the body does not always require it from the diet.

• Non-essential amino acids are produced by the human body from essential amino acids or through normal protein breakdown. These include arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.

All twenty amino acids are equally important for the health of our body. They are the raw materials of peptides and proteins.

Standard abbreviations for amino acids are given in two forms: one-letter and three-letter codes.

A - Ala - alanine
C - Cys - cysteine
D - Asp - aspartic acid
E - Glu - glutamic acid
F - Phe - phenylalanine
G - Gly - glycine
H - His - histidine
I - Ile - isoleucine
K - Lys - lysine
L - Leu - leucine
M - Met - methionine
N - Asn - asparagine
P - Pro - proline
Q - Gln - glutamine
R - Arg - arginine
S - Ser - serine
T - Thr - threonine
V - Val - valine
W - Trp - tryptophan
Y - Tyr - tyrosine

D- and L-Forms of Amino Acids

Amino acids exist in either D (right-handed) or L (left-handed) form. Most amino acids found in nature (and all in human cells) are in the L-form.

In general, all amino acids except glycine also exist as the mirror image of the L-form. This mirror image is called the D-form.

When referring to the naturally occurring L-form, the designation "L" can be omitted, but the designation "D" must always be indicated.

D-amino acids naturally occur in bacterial cell walls and are used in some synthetic peptides to make the peptide more stable and resistant to degradation.

Amino Acid + Amino Acid = Peptide

Amino acids are linked by a so-called "peptide bond". A "peptide bond" is a bond in which the nitrogen atom of one amino acid (from the amino group (NH2)) is bound to the carbon atom of the carboxyl group (COOH) of another amino acid.

During the process of creating this bond, a molecule of water is released. This is called a condensation reaction. The resulting CO-NH bond is called a peptide bond, and the resulting molecule is called an amide.

On the diagram below, note that oxygen and hydrogen (OH) leave from the COOH group, and hydrogen (H) leaves from the NH2 group. This produces H2O, a molecule of water that is not part of the newly formed peptide.

Peptide bond
Through this reaction, a peptide bond is formed between two amino acids, creating a peptide. Such a peptide (composed of two amino acids) can be called a dipeptide.

This process can be repeated using the twenty amino acids as starting material to create longer peptide chains. Sometimes peptide chains consist of fifty to one hundred amino acids, and then they are called polypeptides. A peptide chain with more than 100 amino acids is often called a protein.

GHRP-6

GHRP-6 is a peptide consisting of only six amino acids. Its structure is often written as His-Trp-Ala-Trp-DPhe-Lys-NH2.

Note that the carboxyl group (COOH) is in the first position and is usually not written. The amino group (NH2) is written at the last position.

The "main part," or the section that distinguishes GHRP-6, is the sequence in the middle of histidine, which is bound to the "D" form of tryptophan, bound to alanine, bound to tryptophan, bound to the "D" form of phenylalanine, which is bound to lysine.

Peptide bonds are formed by condensation of water (H2O) (removal of water). Conversely, peptide bonds can be broken down by hydrolysis (adding water).

Amino Acid Structures of Peptides Described in This Article

Growth Hormone Releasing Peptides (GHRP) – initiators of GH pulses

• GHRP-6 (His-DTrp-Ala-Trp-DPhe-Lys-NH2)

• GHRP-2 (DAla-D-2-Nal-Ala-Trp-DPhe-Lys-NH2)

• hexarelin (His-D-2-methyl-Trp-Ala-Trp-DPhe-Lys-NH2)

• ipamorelin (Aib-His-D-2-Nal-DPhe-Lys-NH2) – Ref-1

Notes:
Aib = aminoisobutyric acid
D-2-Nal = "D-form" of 2’-naphthylalanine

Growth Hormone Releasing Hormone (GHRH) – amplifies pulse initiated by GHRP

• Growth Hormone Releasing Hormone (GHRH) known as GRF(1-44) (…) = half-life "less than 10 minutes," perhaps only 5 minutes. – Ref-2

• GRF(1-29), known as sermorelin (…) – biologically active part of GHRH with 44 amino acids = half-life "less than 10 minutes," perhaps only 5 minutes. – Ref-3

Longer-lasting analogues of GRF(1-29):

• D-Ala2 GRF(1-29) (…) = half-life approaching 10 minutes – Ref-4

• modified GRF(1-29) or CJC-1295 without DAC = half-life at least 30 minutes

• CJC-1295 (…) = half-life measured in days

Notes:
Lys = linker to “Drug Affinity Complex” (maleimidopropionyl)

Pulsatile GH Secretion

"Since GH is released in a pulsatile manner and higher growth hormone levels are observed 15 to 30 minutes after subcutaneous administration of GH-RH analogues, hydrolysis by trypsin-like enzymes cannot affect the resulting stimulation."
(Potent Trypsin-resistant hGH-RH Analogues, JAN IZDEBSKI, J. Peptide Sci. 10: 524–529 (2004))

The analogue in the above-mentioned study resisted degradation for 30 minutes. From this citation, it follows that if your analogue lasts 30 minutes, it has reached the potential for one pulse.

Since another pulse will not be generated for about 2.5 to 3 hours, analogues lasting longer than 30 minutes, even up to 3 hours, will not be any more effective.

You would need an analogue that keeps Growth Hormone Releasing Hormone active for more than 3 hours to trigger a second pulse.

Otherwise, you maximize GH release by administering a 30-minute analogue every 3 hours. OR you can simply use an analogue such as CJC-1295, which remains in the body for several days and, after just one dose, stimulates multiple GH pulses per day for several days. 

References / Links

1. "lack of effect on ACTH and cortisol plasma levels" - Ipamorelin, the first selective growth hormone secretagogue , K Raun, European Journal of Endocrinology, 1996 ročník139, číslo 5, 552-561 PubMed
2. Rapid enzymatic degradation of growth hormone-releasing hormone by plasma in vitro and in vivo to a biologically inactive product cleaved at the NH2 terminus, Frohman LA, J Clin Invest. 1986 78:906–913 a Incorporation of D-Ala2 in Growth Hormone-Releasing Hormone-( l-29)-NH2 Increases the Half-Life and Decreases Metabolic Clearance in Normal Men, STEVEN SOULE, Journal of Clinical Endocrinology and Metabolism 1994 ročník 79, č. 4 PubMed
3. Rapid enzymatic degradation of growth hormone-releasing hormone by plasma in vitro and in vivo to a biologically inactive product cleaved at the NH2 terminus, Frohman LA, J Clin Invest. 1986 78:906–913 a Incorporation of D-Ala2 in Growth Hormone-Releasing Hormone-( l-29)-NH2 Increases the Half-Life and Decreases Metabolic Clearance in Normal Men, STEVEN SOULE, Journal of Clinical Endocrinology and Metabolism 1994 ročník 79, č. 4 PubMed
4. Incorporation of D-Ala2 in Growth Hormone-Releasing Hormone-( l-29)-NH2 Increases the Half-Life and Decreases Metabolic Clearance in Normal Men, STEVEN SOULE, Journal of Clinical Endocrinology and Metabolism 1994 ročník. 79, č. 4 PubMed