Write a 100-350 word essay about the enzyme human GMPPB. Provide information related to enzyme: i) function, ii) reaction pathways, ii) Location, and iv) diseases. In writing include these four subheadings

### Function
Human GMPPB, or mannose-1-phosphate guanyltransferase beta, is an enzyme crucial for the biosynthesis of GDP-mannose, a key nucleotide sugar involved in glycosylation. GMPPB catalyzes the conversion of mannose-1-phosphate and GTP into GDP-mannose, which is essential for the glycosylation of proteins and lipids. This process is vital for proper cellular function, including protein folding, stability, and cell signaling.

### Reaction Pathways
GMPPB operates within the nucleotide sugar biosynthesis pathway, specifically responsible for the production of GDP-mannose. The reaction catalyzed by GMPPB—combining mannose-1-phosphate with GTP to produce GDP-mannose—is a critical step in the pathway. GDP-mannose is a donor substrate for the glycosylation of proteins and lipids, including the synthesis of glycoproteins and glycolipids, which are essential components of cell membranes and are involved in various cellular processes such as adhesion, signaling, and immune response.

### Location
GMPPB is localized in the cytoplasm, where it performs its enzymatic function as part of the broader glycosylation machinery. The cytoplasmic location allows GMPPB to interact with mannose-1-phosphate and GTP, facilitating the production of GDP-mannose. This product is then transported to the Golgi apparatus and the endoplasmic reticulum, where it is used in glycosylation processes that are critical for proper protein and lipid function.

### Diseases
Mutations in the GMPPB gene are associated with a group of neuromuscular disorders known as congenital disorders of glycosylation (CDG). These disorders result from defects in glycosylation processes, leading to a wide range of symptoms, including muscle weakness, developmental delays, and intellectual disabilities. Additionally, mutations in GMPPB have been linked to limb-girdle muscular dystrophy (LGMD) and congenital myasthenic syndromes (CMS), where patients experience progressive muscle weakness and fatigue. These diseases highlight the critical role of GMPPB in maintaining proper glycosylation, which is essential for the structural integrity and function of muscle cells and other tissues.